Admin access to global canned responses

This pull request enables admins to manage the global canned responses without having to manually enter the correct URLs for doing so. It works by setting :global=>true when invoking allowed_to?. Unfortunately, this does not work for link_to_if_authorized, so I have to override that and also authorize_for.

I know you might not be interested in merging this and instead want upstream to fix this, but for the pragmatic, this is at least a proof-of-concept on how to get this working for now.