Move from SQLite to something else, like MySQL

[cor]

SQLite won't scale at all, and will be very bad at handeling multiple users at the same time.
We need to start using something else, like MySQL.

[codevbus]

Will need to decide and implement which database to use as this will determine what methods are used to prevent injection vulnerabilities.

[cor]

Good point, we can however implement input validation (i. e. check if there are semicolons in the username and such) already, because this won't be affected by the database type we choose since the SQL syntax is 99% the same.

[cor]

We'll move this to a dedicated docker container, see woording/woording-db