Bring up the room confirmation dialog in more situations

This super helpful and nice dialog went mostly unused, especially if you
only joined links to rooms through matrix.to or within NeoChat itself.

The reason why this never showed up for said links is because we were
greedily overwriting the "action". I don't think the code made solid
sense anyway, so I redone it a bit so there's a clearer
"join_confirmed" our UI uses. I added a test case to showcase this
working.

src/app/qml/JoinRoomDialog.qml

@@ -66,7 +66,7 @@ Kirigami.Dialog {
             text: i18nc("@action:button", "Join room")
             icon.name: "irc-join-channel"
             onClicked: {
-                RoomManager.resolveResource(root.room, "join");
+                RoomManager.resolveResource(root.room, "join_confirmed");
                 root.close();
             }
         }