* Add 3 new buttons
- System shutdown button
- System reboot button
- Octoprint restart button
* Enable buttons by default
* Add tests
* Fix tests
* Remove accidentally committed unused code
* Add RESTART device class to RestartOctoprint and RebootSystem buttons
* Apply suggestions to octoprint test_button
* Freeze time for OctoPrint button tests
* Make new button base class to prevent implementing the availability check multiple times