add option to mark entire group conversation as read

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

.../desktop/forum/conversation/ForumConversationViewModel.kt

...ect/briar/desktop/forum/conversation/ForumDropdownMenu.kt

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

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

...tegroup/conversation/PrivateGroupConversationViewModel.kt

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

...readedgroup/conversation/ThreadedConversationViewModel.kt

...adedgroup/conversation/ThreadedGroupConversationScreen.kt

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

...esktop/src/main/resources/strings/BriarDesktop.properties