Revert "Remove is_numeric check from Title::checkUserBlock"

Breaks unit testing (see bug 47031). The commit managed to land in
the branch because of a bug in Jenkins (bug 46723).

This reverts commit 8cc0b601aa2db6db09ac0e4d70847293d75875aa

Change-Id: I4b3fadccaae9c35964a0c47d63b22c4f35148a24
Merge "Revert "Remove is_numeric check from Title::checkUserBlock""