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.
Because several of the tests use it to assert behaviour. Those lines don't show up in this diff as I haven't changed them.