See for reference:
- https://dev.mysql.com/doc/refman/8.0/en/connecting-using-uri-or-key-value-pairs.html#connecting-using-uri
- https://symfony.com/doc/current/doctrine.html
but note that we must use `rawurlencode` instead of `urlencode` which
differ in how they encode a space (as tested).

Fixes: #2651
Closes: #2502 as this is likely fixed but I couldn't reproduce it

Use URL encoding in DATABASE_URL and return mysql_options
as an array (via ugly global variable), so each element
in it can be separately added to the command line using
`@` for expansion.

Because of changing the script to bash, also reverts some
of the changes in 483200a.

The domjudge mysql user should be created by our setup
scripts, so that we test these and need to set the password
only in one place.

Also don't explicitly pass root user/password to dj_setup_database
script. It will infer it from `~/.my.cnf`.

Rename mysql_root to mysql_log helper to clarify behaviour.

This is consistent with all the other functions.