Please consider switching to bootstrap4

Hi, so far Postorious includes/depends on bootstrap3. So far both series get releases, but already can be seen that the work on v4 reduces resources on v3 so maintenance updates can be somewhat late.

Going forward it seems wise to move from bootstrap3 to bootstrap4. Yet OTOH I can't really estimate how hard (or not) that is for mailman3, therefore I wanted to open this issue to discuss and track that.

There even is a v5 in the pipe, but nothing that can be used yet.

P.S. I'll open the same request for Hyperkitty and reference them.