Fix imgur check for disabled domain listings

The current method of checking whether the domain is from imgur for
disabling vote listings (and displaying the notification about them
being disabled) is slightly incorrect, and causes (generally spam)
domains like iimgur.com or 1imgur.com to get caught as well.

This changes to use the is_subdomain() function from utils to prevent
false positives from the check.
Clarify blank score notes in user preferences

Changed ambiguous 'blank for none' to the more descriptive 'leave
blank to show all (submissions/comments)' for the score thresholds