* Use Babel for LuaTeX. There are a number of bugs in Polyglossia under LuaLaTeX with common languages, e.g. <https://github.com/reutenauer/polyglossia/issues/182>.
* Load xcolor. The `xcolor` package must be loaded before the `footnote` package, which we load to fix foonotes in tables. Closes#4861.
* Load xurl if available. This breaks URLs in more locations. Only available with TeX Live 2018 and later.
* Remove obsolete fixltx2e package
* Reindent. Use two spaces to be consistent internally and with other templates.
* Use `bookmark` if available. The `bookmark` package can sometimes correct the levels of headings where `hyperref` cannot: see <https://komascript.de/release3.26>.
* Update LaTeX tests
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