Add email registration restriction (#2350)

* Add ability to limit number of account registrations with same email address

This commit adds server side functionality that will allow operators to
limit the number of accounts that can be registered to the server with
the same email address.

* Updated ini wording

Updated configuration file wording for better description.

servatrice/src/servatrice_database_interface.cpp

@@ -1109,3 +1109,22 @@ QList<ServerInfo_ChatMessage> Servatrice_DatabaseInterface::getMessageLogHistory
 
     return results;
 }
+
+int Servatrice_DatabaseInterface::checkNumberOfUserAccounts(const QString &email)
+{
+    if (!checkSql())
+        return 0;
+
+    QSqlQuery *query = prepareQuery("SELECT count(email) FROM {prefix}_users WHERE email = :user_email");
+    query->bindValue(":user_email", email);
+
+    if (!execSqlQuery(query)) {
+        qDebug("Failed to identify the number of users accounts for users email address: SQL Error");
+        return 0;
+    }
+
+    if (query->next())
+        return query->value(0).toInt();
+
+    return 0;
+}