Add Basic Postgres Full Text Search Support

[jeremyjh]

Adds very simple Postgres Full Text Search support.

Migrates example project to Phoenix 1.7 via a new mix phx.new. Only migrated enough for the tests to work. This is the cause for most of the lines of code, probably I need to split this PR and merge that migration separately.

Tasks needed in this branch:

• Get CI working with updated Elixir/Erlang and Phoenix versions (tests pass fine locally).
• Consider making the API more flexible, such as support plain_to_tsquery, websearch_to_tsquery as well as the current implementation that provides prefix search support missing in those options, but lacks other features. Maybe the option should accept a module that implements a callback so it could be extended by users.
• Also explore if a combination of full text and trigram search is possible using OR; the idea would be to support both ilike '%term%' and the tsquery approach in the same query, to get the best of both worlds.