server-side citation lookup
This MR adds in CrossRef lookup; a couple of variables are added to the config. There's no front end on this, but there are tests.
Also added in: server-side citation formatting with Citeproc. Testing for that too.
/reference/src/csl/apa.csl
and /reference/src/locales-en-US.xml
are sample style files (from the Zotero site). While we might want to include some style files, this is almost certainly not the way we want to do it – probably this goes in config. Can we allow admins to upload these files themselves?