* Check to make sure usernames with '.' cannot be created

* Add test for instance actor account name conflicts

This makes sure that migration 20190715164535_add_instance_actor
won't fail if there's already an account that is named the same
as the domain (minus the .)

* Put the test into the correct context...

* Add another test to split this into two validations

* Don't delete periods when validating username uniqueness (#11392)

The 20190715164535_add_instance_actor migration fails if there's
already a username similar to the domain name, e.g. if you are
'vulpine.club' and have a user named 'vulpineclub', validation
fails.

Upon further review, usernames with periods are dropped by the
regular expression in the Account class, so we don't need to
worry about it here.

Fixes #11392

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