EPUB template improvements.

Include abstract in default template.

Also ensure that the essential styles needed by
pandoc (`styles.html` partial) are included in the
templates.  This is important for correct formatting
of CSL bibliographies.

Note that much of the styling in `styles.html` will
be ignored for EPUB, because of the conditional on
`document-css`, but if it is desired, you can set
the `document-css` variable.

MANUAL.txt

@@ -2188,6 +2188,11 @@ Currently the following pipes are predefined:
 :   document summary, included in LaTeX, ConTeXt, AsciiDoc, and docx
     documents
 
+`abstract-title`
+:   title of abstract, currently used only in HTML and EPUB.
+    This will be set automatically to a localized value,
+    depending on `lang`, but can be manually overridden.
+
 `keywords`
 :   list of keywords to be included in HTML, PDF, ODT, pptx, docx
     and AsciiDoc metadata; repeat as for `author`, above

data/templates/default.epub2

@@ -6,11 +6,12 @@
   <meta http-equiv="Content-Style-Type" content="text/css" />
   <meta name="generator" content="pandoc" />
   <title>$pagetitle$</title>
-$if(highlighting-css)$
   <style type="text/css">
-$highlighting-css$
-  </style>
+    $styles.html()$
+$if(highlighting-css)$
+    $highlighting-css$
 $endif$
+  </style>
 $for(css)$
   <link rel="stylesheet" type="text/css" href="$css$" />
 $endfor$
@@ -45,6 +46,12 @@ $endif$
 $if(rights)$
   <div class="rights">$rights$</div>
 $endif$
+$if(abstract)$
+<div class="abstract">
+<div class="abstract-title">$abstract-title$</div>
+$abstract$
+</div>
+$endif$
 $else$
 $if(coverpage)$
 <div id="cover-image">

data/templates/default.epub3

@@ -5,11 +5,12 @@
   <meta charset="utf-8" />
   <meta name="generator" content="pandoc" />
   <title>$pagetitle$</title>
-$if(highlighting-css)$
   <style>
-$highlighting-css$
-  </style>
+    $styles.html()$
+$if(highlighting-css)$
+    $highlighting-css$
 $endif$
+  </style>
 $for(css)$
   <link rel="stylesheet" type="text/css" href="$css$" />
 $endfor$
@@ -45,6 +46,12 @@ $endif$
 $if(rights)$
   <div class="rights">$rights$</div>
 $endif$
+$if(abstract)$
+<div class="abstract">
+<div class="abstract-title">$abstract-title$</div>
+$abstract$
+</div>
+$endif$
 </section>
 $else$
 $if(coverpage)$