[Server] Serve account registration from the database interface (#7841)

cmdRegisterAccount and cmdActivateAccount still called
Servatrice_DatabaseInterface for username validation, user existence, the
per-email account limit, ban checks, registration and activation, and
cmdRegisterAccount issued the activation-email insert inline. Add
addActivationEmail to Server_DatabaseInterface with a false default, implement
it with the same statement and bind value, and let both commands ask the
injected interface. Validation order and every response code are unchanged.

servatrice/src/servatrice_database_interface.cpp

@@ -359,6 +359,13 @@ bool Servatrice_DatabaseInterface::registerUser(const QString &userName,
     return true;
 }
 
+bool Servatrice_DatabaseInterface::addActivationEmail(const QString &userName)
+{
+    QSqlQuery *query = prepareQuery("insert into {prefix}_activation_emails (name) values(:name)");
+    query->bindValue(":name", userName);
+    return execSqlQuery(query);
+}
+
 bool Servatrice_DatabaseInterface::activateUser(const QString &userName, const QString &token)
 {
     if (!checkSql()) {