Create sessions through the user session manager

Sign-in now creates and stores its session through the
UserSessionManagerProtocol rather than the UserSessionStore, so the
AppCoordinator no longer holds the store.

- The manager forwards clientSessionDelegate and
  userSession(for:sessionDirectories:passphrase:) to its store. As with
  restoring, the new session isn't registered until the AppCoordinator
  adds it.
- AuthenticationService takes the manager, for both sign-in and soft
  logout.
- The store mock's configuration moves to the manager mock.

UnitTests/Sources/ServerSelectionScreenViewModelTests.swift

@@ -363,7 +363,7 @@ struct ServerSelectionScreenViewModelTests {
         client = factoryConfiguration.homeserverClients["matrix.org"]
         clientFactory = ClientFactoryMock(factoryConfiguration)
         
-        service = AuthenticationService(userSessionStore: UserSessionStoreMock(.init()),
+        service = AuthenticationService(userSessionManager: UserSessionManagerMock(.init()),
                                         encryptionKeyProvider: EncryptionKeyProvider(),
                                         classicAppManager: nil,
                                         clientFactory: clientFactory,