Use FormCard on the invitation page

It now looks a bit nicer, and I made sure everything scales down to
mobile as well.

src/app/qml/RoomPage.qml

@@ -127,10 +127,9 @@ Kirigami.Page {
     Loader {
         id: invitationLoader
         active: root.currentRoom && root.currentRoom.isInvite
-        anchors.centerIn: parent
+        anchors.fill: parent
         sourceComponent: InvitationView {
             currentRoom: root.currentRoom
-            anchors.centerIn: parent
         }
     }