LaTeX template: Render \subtitle (#5213)

Addresses closed issues #4675, #3896, #1327.

This renders `\subtitle` using the `etoolbox` package if `\subtitle` is not already
defined by the documentclass (as it is in beamer, KOMA, memoir classes).

 Based on an example from Enrico Gregorio, <https://tex.stackexchange.com/a/50186>.

Update list of LaTeX packages.

Note that `etoolbox` must be loaded outside the command definition.
Putting it inside causes an error if `\title` is placed after `\begin{document}`.
It's already loaded on LuaLaTeX/XeLaTeX in any case.

MANUAL.txt

@@ -166,47 +166,46 @@ pandoc does not require them to be present:
 [`parskip`] (for better inter-paragraph spaces),
 [`xurl`] (for better line breaks in URLs),
 [`bookmark`] (for better PDF bookmarks),
-[`footnote`] (to fix footnotes in tables).
+and [`footnote`] (to fix footnotes in tables).
 
+[TeX Live]: http://www.tug.org/texlive/
 [`amsfonts`]: https://ctan.org/pkg/amsfonts
 [`amsmath`]: https://ctan.org/pkg/amsmath
-[`lm`]: https://ctan.org/pkg/lm
-[`ifxetex`]: https://ctan.org/pkg/ifxetex
-[`ifluatex`]: https://ctan.org/pkg/ifluatex
-[`listings`]: https://ctan.org/pkg/listings
-[`fancyvrb`]: https://ctan.org/pkg/fancyvrb
-[`longtable`]: https://ctan.org/pkg/longtable
-[`booktabs`]: https://ctan.org/pkg/booktabs
-[`graphicx`]: https://ctan.org/pkg/graphicx
-[`grffile`]: https://ctan.org/pkg/grffile
-[`geometry`]: https://ctan.org/pkg/geometry
-[`setspace`]: https://ctan.org/pkg/setspace
-[`xecjk`]: https://ctan.org/pkg/xecjk
-[`hyperref`]: https://ctan.org/pkg/hyperref
-[`ulem`]: https://ctan.org/pkg/ulem
 [`babel`]: https://ctan.org/pkg/babel
-[`bidi`]: https://ctan.org/pkg/bidi
-[`mathspec`]: https://ctan.org/pkg/mathspec
-[`unicode-math`]: https://ctan.org/pkg/unicode-math
-[`polyglossia`]: https://ctan.org/pkg/polyglossia
-[`fontspec`]: https://ctan.org/pkg/fontspec
-[`upquote`]: https://ctan.org/pkg/upquote
-[`microtype`]: https://ctan.org/pkg/microtype
-[`csquotes`]: https://ctan.org/pkg/csquotes
-[`natbib`]: https://ctan.org/pkg/natbib
+[`biber`]: https://ctan.org/pkg/biber
 [`biblatex`]: https://ctan.org/pkg/biblatex
 [`bibtex`]: https://ctan.org/pkg/bibtex
-[`biber`]: https://ctan.org/pkg/biber
-[TeX Live]: http://www.tug.org/texlive/
-[`wkhtmltopdf`]: https://wkhtmltopdf.org
-[`weasyprint`]: http://weasyprint.org
-[`prince`]: https://www.princexml.com/
-[`upquote`]: https://ctan.org/pkg/upquote
-[`microtype`]: https://ctan.org/pkg/microtype
-[`parskip`]: https://ctan.org/pkg/parskip
-[`xurl`]: https://ctan.org/pkg/xurl
+[`bidi`]: https://ctan.org/pkg/bidi
 [`bookmark`]: https://ctan.org/pkg/bookmark
+[`booktabs`]: https://ctan.org/pkg/booktabs
+[`csquotes`]: https://ctan.org/pkg/csquotes
+[`fancyvrb`]: https://ctan.org/pkg/fancyvrb
+[`fontspec`]: https://ctan.org/pkg/fontspec
 [`footnote`]: https://ctan.org/pkg/footnote
+[`geometry`]: https://ctan.org/pkg/geometry
+[`graphicx`]: https://ctan.org/pkg/graphicx
+[`grffile`]: https://ctan.org/pkg/grffile
+[`hyperref`]: https://ctan.org/pkg/hyperref
+[`ifluatex`]: https://ctan.org/pkg/ifluatex
+[`ifxetex`]: https://ctan.org/pkg/ifxetex
+[`listings`]: https://ctan.org/pkg/listings
+[`lm`]: https://ctan.org/pkg/lm
+[`longtable`]: https://ctan.org/pkg/longtable
+[`mathspec`]: https://ctan.org/pkg/mathspec
+[`microtype`]: https://ctan.org/pkg/microtype
+[`natbib`]: https://ctan.org/pkg/natbib
+[`parskip`]: https://ctan.org/pkg/parskip
+[`polyglossia`]: https://ctan.org/pkg/polyglossia
+[`prince`]: https://www.princexml.com/
+[`setspace`]: https://ctan.org/pkg/setspace
+[`ulem`]: https://ctan.org/pkg/ulem
+[`unicode-math`]: https://ctan.org/pkg/unicode-math
+[`upquote`]: https://ctan.org/pkg/upquote
+[`weasyprint`]: http://weasyprint.org
+[`wkhtmltopdf`]: https://wkhtmltopdf.org
+[`xecjk`]: https://ctan.org/pkg/xecjk
+[`xurl`]: https://ctan.org/pkg/xurl
+
 
 
 Reading from the Web
@@ -1361,10 +1360,7 @@ depending on the output format, but include the following:
         ...
 
 `subtitle`
-:   document subtitle, included in HTML, EPUB, LaTeX, ConTeXt, and Word docx;
-    renders in LaTeX only when using a document class that supports
-    `\subtitle`, such as `beamer` or the [KOMA-Script] series (`scrartcl`,
-    `scrreprt`, `scrbook`).[^subtitle]
+:   document subtitle, included in HTML, EPUB, LaTeX, ConTeXt, and Word docx
 
 `institute`
 :   author affiliations (in LaTeX and Beamer only).  Can be a
@@ -1403,15 +1399,6 @@ depending on the output format, but include the following:
 :   JSON representation of all of the document's metadata. Field
     values are transformed to the selected output format.
 
-[^subtitle]: To make `subtitle` work with other LaTeX
-    document classes, you can add the following to `header-includes`:
-
-        \providecommand{\subtitle}[1]{%
-          \usepackage{titling}
-          \posttitle{%
-            \par\large#1\end{center}}
-        }
-
 Language variables
 ------------------
 

data/templates/default.latex

@@ -350,7 +350,15 @@ $if(title)$
 \title{$title$$if(thanks)$\thanks{$thanks$}$endif$}
 $endif$
 $if(subtitle)$
-\providecommand{\subtitle}[1]{}
+$if(beamer)$
+$else$
+\usepackage{etoolbox}
+\makeatletter
+\providecommand{\subtitle}[1]{% add subtitle to \maketitle
+  \apptocmd{\@title}{\par {\large #1}}{}{}
+}
+\makeatother
+$endif$
 \subtitle{$subtitle$}
 $endif$
 $if(author)$