Full-text searching #155

The following would be required to implement full-text searching in The Arsse:

An implementation for SQLite
An implementation for PostgreSQL
An implementation for MySQL
A means of disabling full-text indexing to conserve space or processing time
A basic LIKE-based fallback if the feature is disabled or unavailable for SQLite, already done

The implementations would need to handle not only querying, but also populating the index when articles are added, as well as cleaning it up properly (if necessary) when articles are deleted.

All databases should have an implementation before a release with full-text searching is made.