Make clear that deleting single messages is just about our side

Related to #286

src/main/kotlin/org/briarproject/briar/desktop/conversation/ConversationItemView.kt

@@ -185,7 +185,7 @@ fun ConversationItemView(
             ContextMenuArea(
                 items = {
                     listOf(
-                        ContextMenuItem(i18n("delete")) { onDelete(item.id) }
+                        ContextMenuItem(i18n("conversation.delete.single")) { onDelete(item.id) }
                     )
                 }
             ) {

src/main/resources/strings/BriarDesktop.properties

@@ -39,6 +39,7 @@ conversation.message.unread=Unread messages
 conversation.message.new=New Message
 conversation.delete.all.dialog.title=Confirm Message Deletion
 conversation.delete.all.dialog.message=Are you sure that you want to delete all messages?
+conversation.delete.single=Delete message for me
 conversation.delete.failed.dialog.title=Could not delete all messages
 conversation.delete.failed.dialog.message.ongoing_both=Messages related to ongoing invitations and introductions cannot be deleted until they conclude.
 conversation.delete.failed.dialog.message.ongoing_introductions=Messages related to ongoing introductions cannot be deleted until they conclude.