Allow the app to be configured to bypass the server selection screen. (#4131)

* Make account provider configuration more flexible.

- Change defaultHomeserverAddress to an array of providers (needs UI).
- Add allowOtherAccountProviders to prevent the user from manually entering a provider.

* Refactor QR code scan failures into a common type.

* Validate scanned QR codes against the allowed account providers.

* Hide the login flow on the QR code screen when restricted.

ElementX/Sources/Services/Authentication/AuthenticationService.swift

@@ -36,7 +36,7 @@ class AuthenticationService: AuthenticationServiceProtocol {
         self.appHooks = appHooks
         
         // When updating these, don't forget to update the reset method too.
-        homeserverSubject = .init(LoginHomeserver(address: appSettings.defaultHomeserverAddress, loginMode: .unknown))
+        homeserverSubject = .init(LoginHomeserver(address: appSettings.accountProviders[0], loginMode: .unknown))
         flow = .login
     }
     
@@ -146,7 +146,7 @@ class AuthenticationService: AuthenticationServiceProtocol {
     }
     
     func reset() {
-        homeserverSubject.send(LoginHomeserver(address: appSettings.defaultHomeserverAddress, loginMode: .unknown))
+        homeserverSubject.send(LoginHomeserver(address: appSettings.accountProviders[0], loginMode: .unknown))
         flow = .login
         client = nil
     }