refactor Content-Security-Policy headers building

[nijel]

Proposed changes

• Split up the code to smaller blocks
• Better detect authentication method URLs (fixes Azure AD Backends not working #12302, fixes Automatic configuration of Content Security Policy form-action for Social Auth #12321)
• Avoid using blank hostnames in CSP

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

[codecov]

Codecov Report

Attention: Patch coverage is 73.91304% with 24 lines in your changes missing coverage. Please review.

Project coverage is 91.14%. Comparing base (8f20b96) to head (a16c0bd).
Report is 6 commits behind head on main.

Files	Patch %	Lines
weblate/middleware.py	73.91%	20 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12322      +/-   ##
==========================================
+ Coverage   91.13%   91.14%   +0.01%     
==========================================
  Files         589      589              
  Lines       60164    60198      +34     
  Branches     9515     9523       +8     
==========================================
+ Hits        54828    54870      +42     
+ Misses       3698     3687      -11     
- Partials     1638     1641       +3     

Files	Coverage Δ
weblate/middleware.py	80.56% <73.91%> (+6.85%)	⬆️

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ OfflineGenerationError: You have offline compression enabled but key "e965160c6ba68c40c8c9f1b184bd4b314e42141a2ac7fde7b48... /browse/{path}/ View Issue
• ‼️ OfflineGenerationError: You have offline compression enabled but key "e965160c6ba68c40c8c9f1b184bd4b314e42141a2ac7fde7b48... /browse/{path}/ View Issue
• ‼️ OfflineGenerationError: You have offline compression enabled but key "e965160c6ba68c40c8c9f1b184bd4b314e42141a2ac7fde7b48... /activity/language/month/en/odio-edit/ View Issue
• ‼️ OfflineGenerationError: You have offline compression enabled but key "e965160c6ba68c40c8c9f1b184bd4b314e42141a2ac7fde7b48... /activity/language/month/en/odio-edit/ View Issue
• ‼️ OfflineGenerationError: You have offline compression enabled but key "e965160c6ba68c40c8c9f1b184bd4b314e42141a2ac7fde7b48... /activity/language/month/en/odio-edit/ View Issue

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