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.
Based on your comments on that last diff, I've taken it further: [diff](https://github.com/dddeastanglia/DDDEastAnglia/compare/9b8a4717085281a6b73149df1d235e37b68af1e7...5b482efdaec7101387ce9b7e735a8cd5e5f3b789). See what you think.