Updated authentication methods to login v2

@katowulf - Please review but **do not merge**. I want to get your thoughts on everything up to this point. There is still more work to be done here: testing (I am going to need to updated `mockfirebase` first...), removing Simple Login code, taking authentication out of this user management file, etc. Please provide any feedback on how things are structured though.
Great suggestion @katowulf! I updated `onAuth()` to return a dispose method and as a result I just got rid of the existing `offAuth()` method. We will have to make sure we call this out in our documentation updates.