Add a MySQL or MariaDB subchart

[jessebot]

This will allow users to use a different database which they may wish to use and it is officially support by pixelfed, so it may be less buggy.

Bitnami should have both a mariadb and mysql chart we can source, just as we do for postgresql, but longer term, as Broadcom has plans to make all charts paid down the line, we should find another provider, but that's a separate issue for later.

[jessebot]

Some of the work was done in #48 but realistically, someone will have to spend more time making it work, as I don't have the time right now. Happy to take any PRs that make this chart work better with mariadb or mysql.