Prevents students from accidentally changing their enrollment on login (...

...LMS-2773)

@symbolist 
@dianakhuang 

prevents students from accidentally dropping their enrollment from verified to honor when they login with get parameters requesting a change in enrollment
It was actually always a weird hack, it just didn't work before. Now it's hackier, but it works and there are tests for it. Fixing the hack is outside the scope of this PR, since it's a fairly involved thing that runs all throughout the login/registration workflow.