Verify Element X with an existing Element Classic account. (#5374)

* Read and import the secrets from ClassicAppAccounts.

* Record snapshots.

* Add some documentation, tidy up tests and fix the dismissal of the backup instructions.

* Workaround flakey tests (the fulfilments weren't always firing).

* Allow a custom Classic App deep link URL to be configured.

ElementX/Sources/Screens/Authentication/StartScreen/AuthenticationStartScreenModels.swift

@@ -26,6 +26,9 @@ struct AuthenticationStartScreenViewState: BindableState {
     let showCreateAccountButton: Bool
     let showQRCodeLoginButton: Bool
     
+    enum ClassicAppMode { case welcomeBack(ClassicAppAccount), otherOptions(ClassicAppAccount) }
+    var classicAppMode: ClassicAppMode?
+    
     let hideBrandChrome: Bool
     
     var bindings = AuthenticationStartScreenViewStateBindings()
@@ -62,4 +65,5 @@ enum AuthenticationStartScreenViewAction {
     case continueWithClassic(ClassicAppAccount)
     case otherOptions(ClassicAppAccount)
     case closeOtherOptions(ClassicAppAccount)
+    case openClassicApp
 }