Bootstrap5 migration with UI retouch.

Depends on django-mailman3!188 (merged) See also postorius!769 (merged)

This is a relatively big UI refresh for Hyperkitty where some things have moved and other less used options have been removed. Some duplicate buttons and more importantly, the left navigation buttons have been removed to add more emphasis on the email thread and reduce the wasted space on the left.

The 2nd major motive for this refresh is to try to bring Postorius and Hyperkitty to a single single design language so people don't feel like they are using two separate applications (even though that is Technically true). I don't expect any major users of Hyperkitty who don't also use Postorius (although, reverse might be actually possible!).