Alert the user when there is a system timezone mismatch or their timezone is not set in their profile

The ability for user profile hovercards to show their timezone was implemented in 2.3.5 as part of #12394 (closed). However, in version 2.4.4 (from Flathub) I still can't see most people's local times in their profile card, because they haven't set it manually in their XMPP profile vCard, and most of them never will know they should do that (i.e. most of them don't edit their profile at all).

In https://gultsch.social/@daniel/116233493132952716 I discovered that there seems to be an automated standard way of getting this information: XEP-0202 …which might be good especially as a fallback (though I don't know if it should instead be the primary way of handling it, because... what if someone is travelling? or should the app detect the OS' changed timezone and prompt the user to suggest updating their XMPP profile to match it, instead?). It might be nice if Gajim could make use of this.

This would probably help #12666 (closed) be much more feasible!