Restore 'custom' and 'none' options to background imagery list (closes #5226)
Restore 'custom' and 'none' options to background imagery list
(closes #5226)

openstreetmap/iD,AndreasHae/iD,AndreasHae/iD,ngageoint/hootenanny-ui,digidem/iD,morray/iD,openstreetmap/iD,kartta-labs/iD,morray/iD,AndreasHae/iD,kartta-labs/iD,kartta-labs/iD,ngageoint/hootenanny-ui,digidem/iD,openstreetmap/iD,ngageoint/hootenanny-ui