Jinja2 based templates updated to Bootstrap5

Hello folks! I have updated jinja2 based templates for Roundup to use a modern version of Bootstrap and make them more mobile friendly.
Almost all pages have been redesigned and are now fully responsive. I also added support for the Bootstrap dark theme and theme switching.

You can see some screenshots:

https://issues.roundup-tracker.org/file1794/bootstrap5-dark-theme-read-only-issue-mobile.png
https://issues.roundup-tracker.org/file1795/bootstrap5-light-theme-read-only-issue-mobile.png
https://issues.roundup-tracker.org/file1796/bootstrap5-dark-theme-issue-index-and-login-form-desktop.png
https://issues.roundup-tracker.org/file1797/bootstrap5-light-theme-issue-index-after-login-desktop.png
https://issues.roundup-tracker.org/file1798/bootstrap5-dark-theme-issue-editing-mobile.png
https://issues.roundup-tracker.org/file1799/bootstrap5-dark-theme-issue-index-mobile.png
https://issues.roundup-tracker.org/file1800/bootstrap5-light-theme-navigation-mobile.png
https://issues.roundup-tracker.org/file1801/bootstrap5-dark-theme-issue-searching-desktop.png
https://issues.roundup-tracker.org/file1802/bootstrap5-dark-theme-user-list-desktop.png

The templates are based on the classic schema and are available under the MIT License.
If you are interested, you can download them from the Github repository: https://github.com/savchuk1985/roundup-jinja2-templates

The templates are already usable and much improved over the jinja2 based templates distributed with Roundup. But they are not perfect, so suggestions and patches are welcome.

I would like to suggest these as replacements for the current templates, so if you like them please leave a review here: https://issues.roundup-tracker.org/issue2551308