Remove SQL dependency in the backend

[tomaspalma]

Currently the backend experience is extremely coupled with PostgresSQL, which is the database we use in the production environment.

However, Postgres brings a lot of pain during development specially because our database has a huge amount of information about courses and takes too long to insert.

Also, defining the schema using SQL requires more technical knowledge and is more cumbersome. Since we are using django we can fully use the features of its ORM.

This way, it is easier to use sqlite3 in development and postgresql in environment since we just need to change the driver django is going to use. With sqlite3 we would just need to paste the database file and we would have access to the data way more rapidly.