create common GroupSharingViewModel

...otlin/org/briarproject/briar/desktop/forum/ForumScreen.kt

...ject/briar/desktop/forum/sharing/ForumSharingViewModel.kt

...rproject/briar/desktop/privategroup/PrivateGroupScreen.kt

...tegroup/conversation/PrivateGroupConversationViewModel.kt

...top/privategroup/conversation/PrivateGroupDropdownMenu.kt

...ktop/privategroup/sharing/PrivateGroupSharingViewModel.kt

...readedgroup/conversation/ThreadedConversationViewModel.kt

...adedgroup/conversation/ThreadedGroupConversationScreen.kt

...p/threadedgroup/conversation/ThreadedGroupDropdownMenu.kt

...op/threadedgroup/sharing/ThreadedGroupSharingViewModel.kt

...g/briarproject/briar/desktop/viewmodel/ViewModelModule.kt