Remove is_numeric check from Title::checkUserBlock

This should allow the usernames of administrators such as "7"
to show correctly on permissions error pages.

I extracted the working code from UserBlockedError::__construct
into a separate method Block::getPermissionsError, called from
both places with context provided as an argument.

Additional changes to get the test suite to pass are included.

Bug: 46768
Change-Id: I49d973992a99e03b4e8de112b47b737037a85338