Consolidate user login and authentication code

[nasthagiri]

This PR consolidates the actively used code related to login/logout and user authentication in the platform - as a runway for future immediate work in this area.

Specifically:

• Renamed lms.djangoapps.student_account -> lms.djangoapps.user_authn (as a Django App Plugin)
  • bringing in URLs from lms/common.py into the user_authn app
• Moved following views from lms.djangoapps.student_account -> common.djangoapps.student
  • password_change_request_handler
• Moved following views from lms.djangoapps.student_account -> openedx.core.djangoapps.user_api
  • account_settings
• Moved following views from common.djangoapps.student -> lms.djangoapps.user_authn
  • LogoutView
  • login_user
  • create_account
  • auto_auth
  • (kept email-related code in common.djangoapps.student for now)
• Moved following from common.djangoapps.student -> openedx.core.djangoapps.oauth_dispatch
  • oauth token helpers
• Deleted following unused views
  • verify_user_password (in common.djangoapps.student)
  • login_oauth_token (in common.djangoapps.student)

[robrap]

Big step in the right direction. Thanks.

[robrap]

Approve of what I reviewed once tests are passing. Let me know if you need a re-review on any final changes.

[edx-status-bot]

Your PR has finished running tests. There were no failures.

[edx-pipeline-bot]

EdX Release Notice: This PR has been deployed to the staging environment in preparation for a release to production on Wednesday, September 19, 2018.

[edx-pipeline-bot]

EdX Release Notice: This PR has been deployed to the production environment.

[robrap]

FYI: @dianakhuang: Here is the refactor with a couple of small deletions.