[F] Implement PG search filtering

[timbot1789]

This implements a separate Filtering applicator that supports using the pg_search gem rather than searchkick. It also modifies several models to support pg_search based filtering. Currently pg_search can be activated in an environment by setting an environment variable 'USE_PG_SEARCH' to true.

See the apply_spec.rb for a list of currently supported models

Models that do not need to support pg_search because they already implement custom by_keyword:

• entitlement
• annotation
• reading group
• comment

Some Notes / Important differences from Searchkick:

• pg_search can search through associations, but it's somewhat limited. It can't search more than one association deep, and it doesn't index the association. So we'll want to keep an eye on performance for some association-heavy searches.
• Since pg_search calls into Postgres, not Ruby, all field names passed to it need to be the postgres name for the corresponding column, not the ruby method name. So if we have a column name that we alias in rails with alias name title, we need to pass pg_search name, not title. This also means that fields like title_plaintext are unavailable. They could be implemented as a PG function, however.

Possible Improvements:

• I haven't tried to implement highlighting here, mostly because I haven't seen it in the front end in areas that seemed to be calling into keyword search. pg_search supports it though, so we can add it in another PR.
• I haven't intensely tested around typos, incomplete words, etc

[scryptmouse]

Looking good! I think putting one abstraction in would be beneficial to build off of.

[scryptmouse]

this is continuing to look good!