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.

Bug: 46768
Change-Id: Ic3fa926a5a4c109faff35fffbccb60fb06ea4a18
Merge "Remove is_numeric check from Title::checkUserBlock"