Split up ForumViewModel and don't pass State<>

...n/org/briarproject/briar/desktop/forums/AddForumDialog.kt

...tlin/org/briarproject/briar/desktop/forums/ForumScreen.kt

...n/org/briarproject/briar/desktop/forums/ForumViewModel.kt

...arproject/briar/desktop/forums/GroupConversationScreen.kt

...briarproject/briar/desktop/forums/GroupInputComposable.kt

.../briarproject/briar/desktop/forums/GroupListComposable.kt

...roject/briar/desktop/forums/ThreadedConversationScreen.kt

...ect/briar/desktop/forums/ThreadedConversationViewModel.kt

...briar/desktop/privategroups/ThreadedConversationScreen.kt

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