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

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

lib/pleroma/activity/search.ex

@@ -65,10 +65,17 @@ defp restrict_public(q) do
   end
 
   defp query_with(q, :gin, search_query, :plain) do
+    %{rows: [[tsc]]} =
+      Ecto.Adapters.SQL.query!(
+        Pleroma.Repo,
+        "select current_setting('default_text_search_config')::regconfig::oid;"
+      )
+
     from([a, o] in q,
       where:
         fragment(
-          "to_tsvector(?->>'content') @@ plainto_tsquery(?)",
+          "to_tsvector(?::oid::regconfig, ?->>'content') @@ plainto_tsquery(?)",
+          ^tsc,
           o.data,
           ^search_query
         )
@@ -76,10 +83,17 @@ defp query_with(q, :gin, search_query, :plain) do
   end
 
   defp query_with(q, :gin, search_query, :websearch) do
+    %{rows: [[tsc]]} =
+      Ecto.Adapters.SQL.query!(
+        Pleroma.Repo,
+        "select current_setting('default_text_search_config')::regconfig::oid;"
+      )
+
     from([a, o] in q,
       where:
         fragment(
-          "to_tsvector(?->>'content') @@ websearch_to_tsquery(?)",
+          "to_tsvector(?::oid::regconfig, ?->>'content') @@ websearch_to_tsquery(?)",
+          ^tsc,
           o.data,
           ^search_query
         )