feat(ui): Instance customization for bootstrap 5 templates

[KarenKonou]

This PR updates the custom CSS template used by the appearance customization feature to work with the new Bootstrap 5 templates.

Part of #2916
Fixes #16912

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.71%. Comparing base (a368da8) to head (03e5c48).
⚠️ Report is 149 commits behind head on main.

Files with missing lines	Patch %	Lines
weblate/configuration/templatetags/colors.py	80.00%	1 Missing ⚠️

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[KarenKonou]

This is nearly ready to merge but I think it will need a more complete refactor once the Bootstrap 5 migration is finished as doing this in a way that works for both BS3 and BS5 templates is quite difficult.

One incomplete part is the enforce hamburger option. In BS5 this should be achievable simply by removing the .navbar-expand class which can be easily done with JS but I'm not sure what the best way to trigger the script is. Ideas welcome.

[KarenKonou]

@nijel maybe we can remove the enforce hamburger option for now just to make this mergeable and readd it later?

[nijel]

We can also keep it non-working for now if this is the only thing blocking this. Removing it would remove the existing setting for users, which would be annoying if we plan to reintroduce it.