Make SQL formatting optional in `CoderunnerSQLQuestion`

Due to a some bugs in sqlparse formatted SQL code for complex DDL/DML queries can look very unformatted. Thus, it should be optional to use the SQL formatter. The most consistent way to implement this IMO would be to add a format_sql key to the YAML schema of Coderunner SQL questions with a default value of True. We can then overwrite this for individual questions.