refactor GroupConversationScreen

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

...lin/org/briarproject/briar/desktop/forums/ForumStrings.kt

...otlin/org/briarproject/briar/desktop/forums/PostsState.kt

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

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

...ct/briar/desktop/forums/conversation/ForumDropdownMenu.kt

...roject/briar/desktop/forums/conversation/ForumPostItem.kt

.../briarproject/briar/desktop/group/GroupInputComposable.kt

...otlin/org/briarproject/briar/desktop/group/GroupScreen.kt

...tlin/org/briarproject/briar/desktop/group/GroupStrings.kt

...iar/desktop/group/conversation/GroupConversationScreen.kt

...ect/briar/desktop/group/conversation/GroupDropdownMenu.kt

...iarproject/briar/desktop/group/conversation/ThreadItem.kt

...roject/briar/desktop/group/conversation/ThreadItemView.kt

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

...project/briar/desktop/privategroup/PrivateGroupStrings.kt