Extend rule capabilities in domain blacklist

[dantheta]

Continuing from openrightsgroup/blocked-org-uk#402

We could also extend the domain blacklist feature to support exclusions to the broader patterns. For instance:

.*porn is a pattern blocked by the submission filter, but (for example), .*factsboutporn (a hypothetical public information site) might be something that should be submittable.

The submission filter could be modified to accept allow rules, which would override a broader deny rule.