Adds reboot button

[InBrewJ]

NOTE NOTE NOTE

To make this work an extra volume needs to be mapped in the docker-compose file - in the section relating to the player container:

volumes:
      - /other-volumes-before:...
      - /proc/sysrq-trigger:/sysrq

See the scentrooms docker-compose file (from which the changes in this PR take inspiration)
https://github.com/LUSHDigital/lrpi_scentroom/blob/master/docker-compose.yml#L38-L56

Where is the source code for the frontend?

Associated lrpi_tablet_ui repo / branch: https://github.com/InBrewJ/lrpi_tablet_ui/tree/rpi3_fixes_with_reset_button

What does it look like?

This:

What happens when a therapist pushes the button?

This (which, admittedly, isn't the prettiest thing in the world!):

Will a therapist press it accidentally?

Maybe! Any design / UX improvement points are welcome. This is the basic 'get it working' version