Comparison was downcasing only one side, therefore if previously
existing account had a non-lowercase spelling, it would be ignored
when checking for duplicates.

New rake task `mastodon:maintenance:find_duplicate_usernames` will
help find constraint violations that might have occured from the
presence of this bug.

Bump version to 2.3.3

* Permit dots in usernames with conditions

- Dot cannot be the start or end of username
- a.lice and al.ice are considered the same during sign-up

* Fix regex mixin flags