User is able to drop tables

## Current behaviour:
- Users are currently able to do destructive queries against DBs (`DROP TABLE`, `DELETE`, etc). They are also able to add data (`INSERT`, etc).

## Expected behaviour:
- Users should not be allowed to submit queries that can delete, add or change data. (This can be configured in some databases by creating a readonly user)

## Possible solutions:
Application fixes:
- I'd suggest using the ast from the codeschool `sqlite-parser`, walking the ast with [`sqltraverse`](https://github.com/jdrew1303/sqltraverse) (yup it's a shameless plug 😋 ). That way you can check for subqueries that may cause issues and notify the user.

Databases admin fixes:
- Create a read only user for the application to use:
- MySQL:

```sql
grant select on database_name.* to 'read-only_user_name'@'%' identified by 'password';
```
- PostgreSQL:
```sql
psql databasehere
CREATE USER moodle_readonly WITH ENCRYPTED PASSWORD 'blablablapasswordhere';
GRANT CONNECT ON DATABASE moodle TO moodle_readonly;
GRANT USAGE ON SCHEMA public TO moodle_readonly;
GRANT SELECT ON ALL SEQUENCES IN SCHEMA public TO moodle_readonly;
GRANT SELECT ON ALL TABLES IN SCHEMA public to moodle_readonly;
ALTER DEFAULT PRIVILEGES IN SCHEMA public GRANT SELECT ON TABLES TO moodle_readonly;
```

Start by tracing where submitted SQL is executed and compare application-side filtering with the read-only database-user approach described in the issue. Review sqlite-parser and sqltraverse for handling subqueries, then verify that destructive and data-changing queries are rejected or cannot execute through the configured database access.

Written by the indexing model from the issue text.