Coordinator stops (#207)

* Add `stop` method to `Coordinator`

* Use new stop method to cleanup room screen

* Call stop implicitly when removing a child coordinator

* Revert placeholder avatar font change

* Fix PR remarks

ElementX/Sources/Screens/Authentication/ServerSelection/ServerSelectionCoordinator.swift

@@ -82,6 +82,10 @@ final class ServerSelectionCoordinator: Coordinator, Presentable {
     func toPresentable() -> UIViewController {
         serverSelectionHostingController
     }
+
+    func stop() {
+        stopLoading()
+    }
     
     // MARK: - Private