Add Cmd+Enter keyboard shortcut for sending post

Closes #283

Tusker/Screens/Compose/ComposeView.swift

@@ -231,6 +231,7 @@ struct ComposeView: View {
         } label: {
             Text("Post")
         }
+        .keyboardShortcut(.return, modifiers: .command)
         .disabled(!postButtonEnabled)
     }