Option to power off the backend

Since the server already offers the possibility to wake up backends using WOL, I missed the possibility to put a backend back to sleep again. So I decided to implement the functionality to shut down, suspend, hibernate or reboot the backend. By default, this can only be done from an administrator's account, but you can also make it available to users via the settings.

If you like, you can pull the changes.
If you don't mind, could you attempt to refactor this a bit more? See https://scrutinizer-ci.com/g/Jalle19/xbmc-video-server/inspections/f2346245-2bca-44c3-80a5-b77558b483af. I'm thinking there could be a PowerOffCapability class which methods like `hasCapability()` and `getMethod($action)`, that way you wouldn't need the switch statement and you wouldn't need to hold the success/failure message in a temporary variable.

You can take a look at the other issues Scrutinizer found too (6 in total) and see what you can do about them. It tends to spit out some false positives from time to time (mostly regarding the framework itself which has some invalid phpdoc here and there) but usually it's fairly valid.