Multiple database support

[kaedroho]

1. Support adding search indexes for models in backends other than the "default" one.

2. Potentially allow indexing from one database backend into another. Which could be useful for:

• indexing external read-only databases
• using Sqlite FTS (which supports BM25 ranking) to index content in PostgreSQL (which doesn't)

This could also be useful for reducing our build matrix quite a bit, as we could avoid running separate CI builds for every database backend.

[gamesbook]

By 1., would this include MySQL which is a widely used database?

[kaedroho]

MySQL was supported in the Wagtail version, but I couldn't get the tests to pass (after hours of trying) so ended up having to remove it.

I would be happy to accept a PR if anyone was able to integrate it!