Switch from postgres to mariadb

We currently use the default database for onlyoffice, postgres. This has the disadvantage that upgrading postgres to a higher major version requires some actions that are hard to do in a cloud environment, and even harder to automate. For that reason we want to replace it by mariadb, see stackspin#1136 (closed).

Also, nextcloud already has a mariadb instance for its own database, for we can probably reuse that one for onlyoffice, reducing our number of components.