make bad-URL-regexp configurable

this is needed for the digia-internal sanity bot.

on the downside, local sanity checking won't complain about url abuse
anymore (unless the user configures it), but that seems somewhat minor
to me.

Change-Id: Ibbcd2b8ef79668d7f7c7c3a9aa6702c7599c4db1
Reviewed-by: Orgad Shaneh <orgads@gmail.com>