Don't delete periods when validating username uniqueness (#11392) (#11400)

 Don't delete periods when validating username uniqueness (#11392) (#11400)

* 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