Fixed: A follow-up to the source rework

[ahankinson]

This is a follow-up on the source / institution re-work.

• There were a few places where the source siglum was still being used, so I fixed this. I'll keep looking.
• Optimized some admin queryset with select_related
• The check for what groups a user was in was triggering several SQL queries. A new template filter was added that takes a comma-separated string and uses that in a single filter() query.
• The source browse chants view was doing a lot of work before a simple check that would trigger a 404. This check was moved up and simplified (It no longer checks the queryset with in but just checks the segment id matches the expected one).
• The pagination custom tags were also slowing down the page loads, so they were optimized as well.

[dchiller]

@ahankinson Looks good! Realize you didn't explicitly request review yet so lmk if this is good to merge or I was premature...

[ahankinson]

Yeah a merge is okay.