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