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/Application/Navigation/NavigationRootCoordinator.swift

@@ -20,13 +20,12 @@ class NavigationRootCoordinator: ObservableObject, CoordinatorProtocol, CustomSt
     @Published fileprivate var rootModule: NavigationModule? {
         didSet {
             if let oldValue {
-                oldValue.coordinator.stop()
-                oldValue.dismissalCallback?()
+                oldValue.tearDown()
             }
             
             if let rootModule {
                 logPresentationChange("Set root", rootModule)
-                rootModule.coordinator.start()
+                rootModule.coordinator?.start()
             }
         }
     }
@@ -66,7 +65,9 @@ class NavigationRootCoordinator: ObservableObject, CoordinatorProtocol, CustomSt
     // MARK: - Private
     
     private func logPresentationChange(_ change: String, _ module: NavigationModule) {
-        MXLog.info("\(self) \(change): \(module.coordinator)")
+        if let coordinator = module.coordinator {
+            MXLog.info("\(self) \(change): \(coordinator)")
+        }
     }
 }
 
@@ -75,7 +76,7 @@ private struct NavigationRootCoordinatorView: View {
     
     var body: some View {
         ZStack {
-            rootCoordinator.rootModule?.coordinator.toPresentable()
+            rootCoordinator.rootModule?.coordinator?.toPresentable()
         }
         .animation(.elementDefault, value: rootCoordinator.rootModule)
     }