Add instance and content urls as separate entries for the Lightbox Repr

AB-1789
Add an Id attribute to the LightboxItem

...as described by the documentation at
https://docs.google.com/a/bright-interactive.co.uk/document/d/1QDMWMaItZjhacPLncUDzi6s2y05XcF0VpkhhaJN0kAE/edit#

AB-1789
Add asset in lightbox repr

It is used to get and delete assets from lightboxes

AB-1789
Add LightboxAssetRepr to represent assets in lightbox

Basically just adds the asset in lightbox url

AB-1789
Merge pull request #2 from brightinteractive/saint-gobain

Changes to the Rest API reprs to use for applying the changes for St Gobain