Optimize attachment full text search

[vlymar]

When an attachment is uploaded, we attempt to extract the text and store it in the attachments table. That text is then used for full text search when filtering in /opportunities. This triggers very expensive joins, aggregations, and to_tsvector function calls which collectively consume
large amounts of memory.

To better support searching through lots of large attachments we should find another approach to FTS other than pg_search's "associated_against" feature, which is not ideal for this use case.

I think it would make sense to create a dedicated search index and use pg_search's "multisearch" approach. We'd possibly have to make some changes to, or work around Filterer code.