Introduce SQLAlchemy database uris for configuring the admin and roundcube database.
Remove the database configuration option from the setup utility. Using a different
database system than SQLite is not necessary for Mailu. The Mailu database generally
contains static data.

The usage of the *DB_* environment variables is deprecated now.
They can still be used in the release after Mailu 1.9, but will be removed
after that version. This means it will be removed from master after the upcoming
Mailu release.

To start using the new environment variables, all *DB_* environment variables must be changed to:
SQLALCHEMY_DATABASE_URI=<SQLAlchemy database URL>
SQLALCHEMY_DATABASE_URI_ROUNDCUBE=<Roundcube database URL>

If no URI is specified, SQLite is used with these settings:
SQLALCHEMY_DATABASE_URI=sqlite:////data/main.db
SQLALCHEMY_DATABASE_URI_ROUNDCUBE=sqlite:////data/roundcube.db