Think about abstractions over systemd

[stefwalter]

The current Services UI is almost pure systemd, which is nice and powerful, but we might be able to put some nice abstractions over it with little effort.

For example, the user can choose the default target by enabling exactly one of the targets that install themselves as an alias for "default.target". Once you know this, it is fairly easy to do with raw systemd. Still, we can make a "Default Server Profile" page that presents these targets as radio buttons.

[stefwalter]

@mvollmer implemented abstractions over systemd. As far as the services page, that will stay a systemd page, and any alternatives should be implemented as a separate module.