Based on #2480
This actually indexes events based on their event type. They are removed
from the index if we receive a `m.room.redaction` event on the
`OutputRoomEvent` stream.
An admin endpoint is added to reindex all existing events.
Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>
This adds the main component of the fulltext search.
This PR doesn't do anything yet, besides creating an empty fulltextindex
folder if enabled. Indexing events is done in a separate PR.
