Add arxiv target

[musoke]

The preprint target outputs a very plain file without any styling. It is nice when the preprint version of a paper has similar formatting to the published version.
There have been requests and discussions concerning a smooth way to do this:

• Make .tex source of paper available joss#132
• Support generation of LaTeX files without JOSS styling #16 - very similar PR, but with lualatex/pdflatex issues.
• LaTeX style file joss#1161 (comment)
• Retrieve TeX file of published article joss#1229

Some issues were raised, the biggest one being possible confusion about the publication status of a preprint. (See for example openjournals/joss#132 (comment) and openjournals/joss#132 (comment).)

In this PR I have configured pandoc to output a LaTeX file somewhat closer to that which would generate the official PDFs. In order to distinguish it from a published article, this style:

• Removes the journal logo from the header.
• Replaces the DOI and review details in the sidebar with Preprint: Prepared for submission to $journal.title$.
  This is borrowed from JCAP's LaTeX style.
• Removes the self-citation in the footer.

There are other minor differences in fonts and PDF tags because arxiv.org only supports pdflatex, not lualatex:

• remove \pdfvariable omitcidset=*
• remove \setmonofont{Hack}
• remove \usepackage{fontsetup}

Possible issues/room for improvement:

• I haven't tested this in the docker image. I could not build the docker image locally, even on main (possibly due to Cannot apk add - temporary error alpinelinux/docker-alpine#59).
• Should the references go in a .bbl, more in line with arxiv's usual processes?
• Is it acceptable to use the journal logo, DOI, etc for an arxiv submission after an article is accepted?
• The arxiv stamp overlaps slightly with the sidebar.

Here is the PDF preview generated by arxiv:

This PR may not be merged here, but I figured I would put the changes out there in case they are useful to someone.