Rather than copy the same artefact once per page we should only copy
it once per copy of page(s) transaction. Eg if we are to copy a
collection of 5 pages and they all have a block pointing to the same
image we should copy that image only once not 5 times.

behatnotneeded - behat file to come

Change-Id: Iecdde53515cdd9d5ee02918252b486aa0f662fab
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>