Event model (#1512)

[blms]

In this PR

Per #1512:

• Add an event model with M2M relations with documents, people, and places
• Add inline forms to Document, Person and Place admins for associating events
• When adding an event from the admin, require at least one document
  • Set min_num of Documents to 0 when creating an Event directly from a Document page; otherwise, min_num=1
  • Prevent new Events from being created via Person or Place admins, per team's request (also would complicate Event-Documents min_num code if this were allowed!)
• Auto-generate a date range for each event based on associated documents, if possible, but allow override
  • Find the min and max dates, mixing inferred and on-document, from all associated documents, and generate a standardized date range from that

Other:

• Convert standard date display function into static method for use outside of Document model (basically anywhere we have standardized dates/ranges)

Notes

Looking especially for review on the Django admin forms trickery, the standard_date_display function changes, and the documents_date_range function

[codecov[bot]]

Codecov Report

Merging #1562 (ff2e685) into develop (14946ab) will increase coverage by 0.03%. Report is 7 commits behind head on develop. The diff coverage is 100.00%.

:exclamation: Current head ff2e685 differs from pull request most recent head a9b9707. Consider uploading reports for the commit a9b9707 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1562 +/- ## =========================================== + Coverage 98.51% 98.55% +0.03% =========================================== Files 231 234 +3 Lines 12784 13113 +329 =========================================== + Hits 12594 12923 +329 Misses 190 190 ```