Remove the MockUserService

[NicMcPhee]

I think this actually adds a lot of complication, isn't used that much, and is probably the wrong tool for the job. I think that spies and stubs are a better solution and allow us to focus more clearly on what we're trying to test. We don't actually use the MockUserService much, so I think we can just get rid of it, moving the necessary stubs directly into the relevant tests.

If we do this, then I think we can also remove the | null from the constructor in UserService, and drop all the !s in that file that were added because the httpClient could be null.