Split up account migrations to their own function

[fisher60]

Currently these lines are used to retrieve discord OAuth data, check if a user has an existing temp account, and migrate the temp account data to a Friendo account. This function is a bit bloated and it is not clear what it does, functionality should be split up.

As per current convention on this project, it might make sense to add user validation/migration logic to the models.py file in the users app, while keeping the redirect/UI logic in the views.py file.