Room Directory Search should use Text Search on Postgres

This issue has been migrated from [#5747](https://github.com/matrix-org/synapse/issues/5747).

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The room directory search uses substring-in-string for search whereas [text search](https://www.postgresql.org/docs/9.6/textsearch-controls.html) (which will do scoring against actual English* words, and can prioritise matches in the room name vs the topic) might be a good fit here.

The user directory already uses it (even though usernames don't benefit from it as much as rooms would, since on my homeserver at least, it just seems to index the user-part and the domain-part of the user ID and the words from the display name.)

This issue should follow on from #4043.