* Add support for IPv6 only MXes

* Fixed email validator tests

* Add specs for UnreservedUsernameValidator

* Use instance variable

* Add specs for BlackListedEmailValidator

* Use instance variable

In order to implement tests easier, `#select_tags` created.

* Add pending specs of StatusLengthValidator

* Use instance variable

Since dots are not allowed in username,
this spec is no longer needed.

Note that this spec has a pending test about dots in the username,
because allowing it has been reverted for now.

This fixes the StatusLengthValidator tests to use
StatusLengthValidator::MAX_CHARS to derive the length of the strings
tested, and so makes them resilient to changes in MAX_CHARS (such as the
one that made these tests fail, with it being changed from 500 to 512).

This seems a bit odd for some types of tests, but appears to make sense
here, where the testing goes beyond `status.length > MAX_CHARS`.

* Count all URLs in text as 23 characters flat, do not count domain part of usernames

* Add new status text counting logic to web UI