make password validation code more DRY

grammar nit: "less" -> "fewer"

Use a contextmanager to cleanup after manipulating the current request in password_policy tests for external auth

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

for visual indent

LMS-6622

Some users erroneously set their password as their username with the original layout, because the username field directly followed the password field. Users may be accustomed to the common occurrence of a password confirmation field directly following the password field.
To fix the issue, I did the following:
   - Moved the existing username and real name form fields above the password field.
   - Added a validation in the create_account handler in common/djangoapps/student/views.py, which confirms that the password field does not match the username field.

New tests created to check the added functionality.

Add some optional policies around password resets, such as password reuse, forced password resets, allowed frequency of password resets

LMS-1479