Customize the Unity desktop on cluster machines

Add some gsettings override keys to disable the additional
search results in the Unity Lens, and also remove Amazon
from the launcher, and add xmoira instead.
Add dconf locks to disable user switching

Add a "user" profile for dconf, and the "athena" db, which disables
the user switcher in the indicator-session menu
Disable the guest session in lightdm

indicator-session uses the display-manager to determine if guest
login should be allowed, so we disable it.  Fortunately, lightdm
now supports conf.d snippets
Add changelog entries for previous commits