feat(core): Add new SOGoURLCreateAccount option to add a create account link below login form

SoObjects/SOGo/SOGoSystemDefaults.h

@@ -140,6 +140,8 @@ NSComparisonResult languageSort(id el1, id el2, void *context);
 - (BOOL)isURLEncryptionEnabled;
 - (NSString *)urlEncryptionPassphrase;
 
+- (NSString *)urlCreateAccount;
+
 @end
 
 #endif /* SOGOSYSTEMDEFAULTS_H */

SoObjects/SOGo/SOGoSystemDefaults.m

@@ -928,4 +928,9 @@ NSComparisonResult languageSort(id el1, id el2, void *context)
   return disableExport;
 }
 
+- (NSString *)urlCreateAccount
+{
+  return [self stringForKey: @"SOGoURLCreateAccount"];
+}
+
 @end