add "clone group"

[bjoern]

"Clone Group" was added to android/ios for 1.41 on the side without clear requirements for some version.

meanwhile, it turned out several times, it is a very handy feature - and that it is really missing on desktop.

from the original issue:

cloning group offers a pragmatic way to add unverified ppl to verified groups or upgrade to verified groups.

also the "birthday issue", where a 30 ppl group wants to discuss a present for one member is fixed by that.
no need to put the option too much in the way, however [...] when tapped [...] "new group" is opened with a prefilled member list and prefilled name

as it is probably also on desktop low effort, it makes sense to add that even when going for maintenance mode.

ui-wise, right-tap in the chatlist or the three-dot-menu of a chat come to mind; if one is easier than the other, i'd go for that on the first iteration.

see android and ios for some reference (only few lines of changed code)