Support server-side search

[NfNitLoop]

The current client-side search is a bit of a hack. It's a brute-force search back through history, so is O(n) and uses O(n) network requests. (And network traffic, if you haven't got a cached copy of the Items locally.)

SQLite actually supports full text search indexes! Could add an option to enable it, and let servers advertise that they support server-side search, at which point clients can use that.

Use cases:

All

• Sort by search relevance
• Allow users to choose which users' posts to search

Search My (a) Feed

• A user is viewing their feed in the UI.

• Enters a search term in the search filter.

• That UI calls the server-side search to find items that match.

• Order by search relevance

• Only show things in the user's feed. (option to tell server which user's feed to search? Though maybe this is too much a specialization and it should just accept a list of userIDs to search?)

Server Search

• Just search "all" users on a server.
• Should we have a way for users to opt into/out of being globally searchable on servers?
  • Possibly limit it to servers that their profile claims to be hosted on?
  • What are the defaults?
• A way for server admins to further limit what's globally searchable/discoverable?

See also: #11