Add an origin blocking proxy

This proxy prevent registered origins from being visited again. If an origin url is matching a blocking rule, then any attempt to add an Origin, OriginVisit or OriginVisitStatus object targeting this url will be blocked, raising a BlockedOriginException.

This is implemented in a similar fashion than the MaskingProxy, sharing the same management logic as this later.

The url matching rules are, given a checked URL:

• check for an exact match in the blocking rules on:

  1. the given URL
  2. the trimmed URL (if it has a trailing /)
  3. the extension-less URL if it ends with a know suffix (eg. '.git')

• if no exact match is found, look for the best prefix match on split sub-path urls (aka the longest url match in the blocking rules for which the URL starts with the match, splitting on '/')

Notes:

• on top of !1123 (merged)
• the blocking matching logic is not working correctly yet, it does not handle the prefix matching on / only (so a blocking rule of say http://myforge/toto will block http://myforge/toto2 which is not the intended behavior).

Related to swh/meta#5088