Just in case two users go to publish the same layer at around the same
time, avoid saving the record for a user who tries to publishes it after
the first time.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Collect history for changes, and allow browsing / reverting / comparison
within the admin interface.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>