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.
I'm not 100% convinced by this (it's just a bag of `bool`s and doesn't really represent a concept), but we can probably leave it for now. 

A nicer design might be to return an `IEnumerable<LoginMethod>`, perhaps.