Replace custom NAV login authenticator with a series of Django authentication backends

[lunkwill42]

Scope and purpose

Fixes #3625. Dependent on #3624

This explicitly sets AUTHENTICATION_BACKENDS in Django settings to include the standard ModelBackend from Django, and the new LdapBackend implemented in #3624.

It changes the existing implementation of NAV's do_login() view to use Django's authentication mechanism, rather than NAV's custom authenticate() function, making the login flow work with the new authentication backends instead.

However, tests are not updated yet. ATM, the failing tests mostly concern themselves with testing the now-removed nav.web.auth.authenticate() function, so these tests will have to be re-thought (likely, they could be removed and rebuilt as tests for the new ldap backend instead?)

Also, these changes to not consider the REMOTE_USER-based login mechanism, so @hmpf may have to look into that before we can merge this.

All in all, the goal is that this should be mergeable before/without completing a transition to implementing django-allauth in NAV.

Contributor Checklist

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More information about contributing to NAV can be found in the
Hacker's guide to NAV.

• Added a changelog fragment for towncrier
• Added/amended tests for new/changed code
• Added/changed documentation
• Linted/formatted the code with ruff, easiest by using pre-commit
• Wrote the commit message so that the first line continues the sentence "If applied, this commit will ...", starts with a capital letter, does not end with punctuation and is 50 characters or less long. See https://cbea.ms/git-commit/
• Based this pull request on the correct upstream branch: For a patch/bugfix affecting the latest stable version, it should be based on that version's branch (<major>.<minor>.x). For a new feature or other additions, it should be based on master.
• If applicable: Created new issues if this PR does not fix the issue completely/there is further work to be done
• If it's not obvious from a linked issue, described how to interact with NAV in order for a reviewer to observe the effects of this change first-hand (commands, URLs, UI interactions)
• If this results in changes in the UI: Added screenshots of the before and after
• If this adds a new Python source code file: Added the boilerplate header to that file

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[github-actions]

Test results

    15 files      15 suites   14m 15s ⏱️
 2 625 tests  2 618 ✅ 0 💤  7 ❌
10 338 runs  10 296 ✅ 0 💤 42 ❌

For more details on these failures, see this check.

Results for commit c3bb990.

[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.41%. Comparing base (27ba495) to head (c3bb990).

Files with missing lines	Patch %	Lines
python/nav/web/webfront/views.py	84.61%	2 Missing ⚠️

Additional details and impacted files

@@                         Coverage Diff                          @@
##           feature/nav-django-ldap-backend    #3626       +/-   ##
====================================================================
+ Coverage                            18.88%   62.41%   +43.53%     
====================================================================
  Files                                  612      612               
  Lines                                45153    45119       -34     
  Branches                                43       43               
====================================================================
+ Hits                                  8527    28161    +19634     
+ Misses                               36616    16948    -19668     
  Partials                                10       10               

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