feat(core): initial Google Authenticator support for 2FA

UI/PreferencesUI/UIxPreferences.m

@@ -1,6 +1,6 @@
 /* UIxPreferences.m - this file is part of SOGo
  *
- * Copyright (C) 2007-2019 Inverse inc.
+ * Copyright (C) 2007-2020 Inverse inc.
  *
  * This file is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -1021,6 +1021,20 @@ static NSArray *reminderValues = nil;
   return [NSString stringWithString: SOGoVersion];
 }
 
+- (BOOL) isGoogleAuthenticatorEnabled
+{
+#if defined(MFA_CONFIG)
+  return YES;
+#else
+  return NO;
+#endif
+}
+
+- (NSString *) googleAuthenticatorKey
+{
+  return [[context activeUser] googleAuthenticatorKey];
+}
+
 //
 // Used internally
 //