Add full text search for observations

[JWCook]

Continuing from #254 and #341.

Depends on:

• [x] https://github.com/pyinat/pyinaturalist-convert/issues/85
• [x] https://github.com/pyinat/pyinaturalist-convert/issues/100

This requires a new FTS table in SQLite, which should include (for the user's observations only):

• [x] description
• [x] standalone comments
• [x] identification comments
• [x] place guess
• [x] index new observations on save
• [ ] index existing observations (initial setup/reset only?)

Then, update the observation tab:

• [ ] Add search field to filter user observations
• [ ] Add truncated matched text to observation preview card