<pre style="white-space:pre-wrap;width:81ex">Fix possible inconsistencies in tag search (#14906)

Do not downcase the queried tag before passing it to postgres when searching:
- tags are not downcased on creation
- `arel_table[:name].lower.matches(pattern)` generates an ILIKE anyway
- if Postgres and Rails happen to use different case-folding rules,
downcasing before query but not before insertion may mean that some
tags with some casings are not searchable</pre>
https://github.com/tootsuite/mastodon/commit/9870b175b477bbc984fc7945f1ebe07e3f2b0053