Add non-partial index on namespace table

What does this MR do and why?

As discussed here, add an index on namespaces table targeting type and id.

There currently exists a partial index on these columns

CREATE INDEX "index_namespaces_on_type_and_id_partial" ON "public"."namespaces" USING btree (
  "type" COLLATE "pg_catalog"."default" "pg_catalog"."text_ops" ASC NULLS LAST,
  "id" "pg_catalog"."int4_ops" ASC NULLS LAST
) WHERE type IS NOT NULL;

Since we're making all namespaces have a type, such as User or Group, we now need a full index rather than a partial. The partial will be removed in an upcoming MR.

Since namespaces is a large and highly trafficked table, we use the procedure detailed here: Create indexes asynchronously

Related to #342543 (closed)

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