Remove the Shutdown Helper panel applet.

It doesn't work in Jessie, and we have better plans (#8302).
Install and enable our custom shutdown-helper GNOME Shell extension (Closes: #8302, #5684 and #5878).

It adds a Lock Screen and Restart button to the GNOME user menu,
removes the press-Alt-to-turn-shutdown-button-into-Suspend
functionality, and makes Restart and Shutdown immediate, without
further user interaction.

It's based on laserb's gnome-shell-extension-suspend-button:
https://github.com/laserb/gnome-shell-extension-suspend-button
Fix copyrights for shutdown-helper.
Fix link to the code.