Show a user's login methods on the user details page in the admin area

We have had a few people who can't login because they originally used a provider (e.g. GitHub), but are then trying to log in via another mechanism. This change shows a user's available login methods in the admin section, so we can easily point them in the right direction.
Shame that this interface has only one implementation. It might be better as an `IQuery<T, TReturn>`: this is the route I'm navigating with my query objects refactoring branch, at least.