TSVector based search provider

What does this MR do?

This implements support for tsvector-based searches with the new search provider system.

On the DB level this is implemented with a generated column containing a tsvector embedding of the post content, cw, and link name concatenated together.See the example given in the config.

Searches not ordered by date use a cover-density based ranking system to return relevant results.

Currently runs in prod on plasmatrap.com.

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