Is your feature request related to a problem? Please describe.
Users get very confused when they leave a MUC and it “comes back” later, after they restarted their client. It’s also weird when you have to join a MUC every time, before you discover/understand the bookmarking mechanism. Until then, you also have no synchronisation with other clients you may use.

Describe the solution you'd like
Remove all mentions of bookmarks in the UI, and instead handle that as internal details of the protocol. Joining a MUC would automatically add a bookmark, leaving it would automatically set autojoin to false in the bookmark (but keep the bookmark so that it can be easily found again at a later point).

Describe alternatives you've considered
Keeping the UI as is is confusing to everyone but protocol nerds.

Additional context
Other clients like Conversations, Gajim, Dino or poezio (and perhaps other ones) already do so, with great UX success.

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