Use PostgreSQL for full text search

[sunu]

PostgreSQL's full text search capabilities should be good enough to handle our need for basic full text search and filtering.

Additionally, we will be able show search results from all documents a user has access to compared to the current state where we show search results from published documents only. (See #759) Although it is possible to implement document level access control in ElasticSearch through filtering, it's much easier to implement through PostgreSQL.

Switching to PostgreSQL will allow us to drop our dependency on OpenSearch/ElasticSearch and simplify our stack as a result.

[bwbaker1]

@wrynearson Is this necessary? I don't see the need for users to search for all of their documents. Is the bigger issue a need to drop OpenSearch/ElasticSearch?

[sunu]

@bwbaker1 I think this is mostly a nice to have and not something we need to do. It also depends on whether we think #759 is an issue worth fixing or not.