Memory management (#503)

* Correctly tear down the user session on signing out

* Fix session verification <-> user session retain cycle, visible range debouncer leak

* Manually clean up coordinators retained within SwiftUI's NavigationStacks

* Slightly refactor the timeline content menu builder and prevent it from retaining the view model. Cleanup now unnecessarily optional RoomScreenCoordinator instance vars

* Move coordinator dismissal logic to the navigation modules

ElementX/Sources/Screens/RoomScreen/RoomScreenViewModelProtocol.swift

@@ -20,6 +20,4 @@ import Foundation
 protocol RoomScreenViewModelProtocol {
     var callback: ((RoomScreenViewModelAction) -> Void)? { get set }
     var context: RoomScreenViewModelType.Context { get }
-
-    func stop()
 }