Added a configuration flag to force third party auth

[umarmughal824]

This adds a toggle to allow operators to prevent user registration and login via username/password authentication, forcing the platform to only support login and registration using third-party auth such as SAML.

Related: https://github.com/mitodl/edx-platform/issues/208

How to Test:

• Turn on a Third party auth following the instructions in https://edx.readthedocs.io/projects/edx-installing-configuring-and-running/en/latest/configuration/tpa/index.html (which could use an update)

• Set ENABLE_REQUIRE_THIRD_PARTY_AUTH to True in lms/envs/common.py

[blarghmatey]

:+1:

[blarghmatey]

Please do a squash and merge to keep this as one commit so that it's easier to cherry pick into future releases.