fix: validate form in login()

[jennifer-richards]

This prevents 500s if username/password contain null characters. It refactors our custom login() view as a custom LoginView subclass and an associated custom AuthenticationForm subclass.

The following applied to the first commit only (which broke the ability to log in using any email address):
The error is happening in some checks we do prior to the actual authentication check - it's looking to see if the User's password was cleared and whether it has a Person. The actual auth happens in the LoginView.as_view() call, which already checks and rejects attempts involving null characters. If that were to change, this PR would potentially make it possible someone to bypass the checks for a cleared password or missing Person. I don't think that's either likely or harmful, but wanted to point at it.

[codecov]

Codecov Report

Attention: Patch coverage is 82.75862% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 88.91%. Comparing base (187c2c5) to head (003545f).
Report is 213 commits behind head on main.

Files	Patch %	Lines
ietf/ietfauth/views.py	82.75%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7435      +/-   ##
==========================================
- Coverage   88.98%   88.91%   -0.08%     
==========================================
  Files         291      294       +3     
  Lines       40717    41181     +464     
==========================================
+ Hits        36233    36617     +384     
- Misses       4484     4564      +80     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.