Bug: in some cases searching for multiple metatags would cause one
metatag to be ignored. For example, a search for {{user:1 pool:2}} would
be treated as a search for {{pool:2}}.
Cause: we used `ActiveRecord::Relation#merge` to combine two relations,
which was wrong because `merge` doesn't combine `column IN (?)` clauses
correctly. If there are two `column IN (?)` clauses on the same column,
then `#merge` takes only the second clause and ignores the first.
Fix: write our own half-baked `#and` method to work around Rails'
broken-by-design `#merge` method.
ref: https://github.com/rails/rails/issues/33501.
8cbcec285d