Factor out database url / credentials in config

[mahmoud]

Per this phabricator ticket, if we allow the config to also support reading from a separate file, that will allow toolforge admins to rotate db passwords without worrying about modifying our prod config directly.

For ref, QuickCategories passes through kwargs for this: https://github.com/lucaswerkmeister/tool-quickcategories/blob/e27a219ad0b1ebec5d5af0393bc5d6dbc63ea13a/config.yaml.example#L6-L12