Use Django conditional response to prevent mid-air collisions (#965)

[blms]

In this PR

Per #965:

• Add method to compute an Annotation's ETag (md5 hash of content)
• Include ETag in compile method for use in annotation lists (required in order to associate a single annotation with a single ETag when retrieved via a list)
• Add conditional response for POST/DELETE using Django @condition in a mixin, in order to return 412 when computed ETag does not match If-Match header
• Update Tahqiq to version in https://github.com/Princeton-CDH/annotorious-tahqiq/pull/32
• Improve styles for alerts, and make them last longer if they are errors

[codecov[bot]]

Codecov Report

Merging #1565 (993ce7e) into develop (14946ab) will increase coverage by 0.00%. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #1565 +/- ## ======================================== Coverage 98.51% 98.52% ======================================== Files 231 231 Lines 12784 12845 +61 ======================================== + Hits 12594 12655 +61 Misses 190 190 ```