Fix long log-in time after lastSubmission reset

[ValerianClerc]

Currently, when lastSubmission is reset for all users, the first login can be very slow in the case where a user has a lot of codeforces submissions, or simply if the codeforces API is being slow. We don't want to be so tightly dependant on their API's load time for our basic flow.

Ideas for fixing this:

• Separate log-in and update-user-submissions into two separate routes
  • Currently, both of these functionalities are done in a single route, so the UI can't continue with the user data until the submissions are updated and the request has returned