When editing site skin, disable skin background images (Bug 1305308)

Also display confirmation dialogue.

Change-Id: I55b4a85cdeabf14ae39027723a6882456ff13c01
Signed-off-by: Son Nguyen <son.nguyen@catalyst.net.nz>
Merge "When editing site skin, disable skin background images (Bug 1305308)" into 1.9_STABLE