add id field to ChildRef
encode id into content identifier
add optional long id to parent ref
SeriesRef now extends ChildRef
Initialise *Identifiers with ids
SeriesRefs aren't ChildRefs
Merge branch 'mbst-4764-add-series-to-sub-items' into stage

Conflicts:
	src/main/java/org/atlasapi/media/entity/Series.java
	src/main/java/org/atlasapi/media/entity/simple/ContentIdentifier.java