Allow certain users to create "blessed" Via links that bypass the allow-list

See also:

* [Require authentication to create Via links](https://github.com/hypothesis/checkmate/issues/74) (@lyzadanger's original issue that inspired this idea)

Allow certain users to create Via links that are somehow "blessed" so that they bypass the allow-list. For example the links might contain a signature.

"Certain users" could mean all authenticated users, or it could mean only certain somehow "blessed" user accounts, or something else.

If malign links get created in this way, perhaps we could disable them somehow (assuming we find out about it) and then possibly ban the particular user from creating any more blessed links.