Ignore DB caches.

* Partner information: 3rd party-hosted open edX instance, for an edX solutions client (so will need to get some sort of solution merged in)
* Merge deadline: Nov 4th (soft requirement to minimize code drift, we maintain it on the client fork in the meantime)

These caches cause problems when testing, as the DB schemas across forks may diverge from the mainline. As well, these caches are created upon the first test run if they are absent, so there isn't any obvious need for them to be in the repository.
My preference would be:

* the files are removed from the repository
* the `.gitignore` file in the root of the repository includes these paths, so they are not re-added to the repository by accident in the future
* if these files always necessary for the tests to run, they are automatically generated when the tests are run at the start of the test suite
* if these files are sometimes useful for tests, but sometimes not useful (for example, separate tests for when the cache exists vs when the cache does not yet exist), then there is some kind of system to generate these files in the `setUp` for the tests that need them, and the files are deleted in the `tearDown` for those tests

Does all of that make sense?