Add support for non-root MySQL databases

[Grendel7]

Fix for issue #194

[GRBurst]

This should make similar checks as other boolean flags, e.g. comparing to 'true', '1', 'yes'.
So this becomes like
'USE_EXISTING_DB': get_conf('USE_EXISTING_DB', '').lower() in ('true', '1', 'yes')

[oexar]

also the database port is hardcoded

It would be great to be able to change it with something like :

db_port = get_conf('DB_PORT '3306')

[Neamar]

Hey there! So if I understand correctly:

• this PR is working and allows to use a non-root DB user
• it's not merged "for various reasons" (comment on Support MySQL without root access #194), but is actually usable and working?

If so, I'm probably going to try setting it up :)

[mlazzarotto]

Bumping the thread, it's 2023 guys...

[freeplant]

The latest version already support using external database: https://manual.seafile.com/docker/deploy_seafile_with_docker/#use-an-existing-mysql-server

And DB_ROOT_PASSWD is needed during installation. Later, after Seafile is installed, the user seafile will be used to connect to the mysql-server (in conf/seafile.conf). You can remove the DB_ROOT_PASSWD.