This allows adding an important notice to a layer e.g. "this layer is
deprecated, please use layer xyz instead". Only one layer note can be
added through the interface although the data structures allow multiple,
so notes may be added programmatically without disturbing user-added
ones.
With this change we also add a get_absolute_url() function to the
LayerItem model and change the calls to reverse() for layers to use it.
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
