Add easily-accessible buttons to shut down and reboot the device

[ethanjli]

Is your feature request related to a problem? Please describe.
Previously, the device's landing page (which was the Node-RED dashboard's landing page) had buttons for shutting down and rebooting the machine, easily accessible. The new device-portal landing page no longer has such buttons.

This feature request was made by Fabien Lombard

Describe the solution you'd like
Fabien proposed adding buttons on the landing page to shut down and reboot the machine.

Describe alternatives you've considered
I already plan to add a control panel (preferably as part of device-portal for simplicity, or else as a separate app), which would have shutdown and reboot buttons. Additionally, the Cockpit home page has buttons for shutting down and rebooting the system. However, those buttons are not easily accessible. It does seem like we need to have buttons on the landing page.

Additional context
I am not sure how we might implement this without running the device-portal as a privileged container in Docker, running commands on the host system. Maybe there's some dbus API we can call into or a socket we can write to?

[ethanjli]

The relevant D-Bus API to appears to be https://www.freedesktop.org/software/systemd/man/latest/org.freedesktop.login1.html (look for the PowerOff() and Reboot() functions).