Make activity search properly use GIN indexes

The original approach to search in GIN indexes is to use
`to_tsvector(text)` in the WHERE clause of the query. According to
postgres docs [pdoc], this method does not make use of the index,
while `to_tsvector(config, text)` does. This commit changed the
query to use the two-argument `to_tsvector()`.

[pdoc]: https://www.postgresql.org/docs/12/textsearch-tables.html

To obtain the search config in use, we make a query to the db first.
The `::regconfig::oid` hack is needed because Postgrex does not support
regconfig type directly [postgrexbug]. I use the conversion from and to
`oid` instead of `text` because I tested in the actual DB and querying
using the conversion via `text` is slow just as the one-argument
`to_tsvector()` variant.

[postgrexbug]: https://github.com/elixir-ecto/postgrex/issues/502

Backport of: https://git.pleroma.social/pleroma/pleroma/-/merge_requests/3519

Closes: https://git.pleroma.social/pleroma/pleroma/-/issues/2758

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