Add support to see subscription requests pending user confirmation.

This adds support to see subscription requests that are pending user confirmation. Workflow itself follows the same pattern, except that the moderator only has the ability to discard a request.

In theory, this does not prevent a moderator from approving a request, but the UI does not expose that interface just to be able to reuse the same view. Even if a new view were written, it is still possible to subscribe a user without asking for their permission, so I don't see this would be a problem.

Fixes #314 (closed)