Use Django's caching for indexing data caching.

[ShawnMilo]

As an upgrade to #460, use Django's built-in caching (with its expiration support to replace the dicts in search/search_indexes.py.

This will make the code simpler. This may also mean replacing the ALLOW_CACHING setting that was added in that PR in favor of a CACHE_TIMEOUT setting used to populate the Django caching TIMEOUT setting. This would be zero (no caching) by default and could be changed as appropriate in production.

[ShawnMilo]

At @carsongee's recommendation, we should also add tests to this improvement PR which ensure the number of DB queries goes down. This can be accomplished with assertNumQueries.