UserMixin includes methods for which the necessary settings might be missing

As @BeritJanssen pointed out in https://github.com/CentreForDigitalHumanities/backbone-util/pull/1#pullrequestreview-2057248290, it is currently possible to obtain a user mixin that has a `login` method but no `loginUrl`. This is not great from an interface design perspective.

One possible solution would be to make all settings required. However, this would be inconvenient for users who do not need all methods. For example, you might only need `login`, but would still have to specify the `confirmRegistrationUrl` setting.

Another possible solution, suggested by @BeritJanssen in https://github.com/CentreForDigitalHumanities/backbone-util/pull/1#discussion_r1602882951, is to prune any methods that are missing a necessary setting.

Start by reviewing the UserMixin implementation and the settings discussed in pull request #1 and its review comments. The issue presents two possible approaches but does not choose one or name a file or test; done requires agreeing on the interface behavior and verifying that methods cannot remain available without their necessary settings.

Written by the indexing model from the issue text.