New database search backend

The goal of my project is to replace the current default database search backend in Wagtail, which is a dummy backend that comes enabled by default, with a functional one that works on SQLite, PostgreSQL, and MySQL.

My specific goals are:

• Remove the contrib PostgreSQL backend that is currently defined, without making breaking changes that could affect current Wagtail installations.
• Implement a new core search backend that is independent of the specific database system used, which acts as a bridge between the rest of the Wagtail apps and the database-specific backends.
• Implement a specific backend, which is bridged to by the base core backend, for the PostgreSQL, SQLite, and MySQL database systems. It should take advantage of the specific FTS data types and functions that are defined in each one.
• Document the code of the backends, both in the source code and in Wagtail’s documentation.