CMS-2059 Enrich Content with children

Put ImmutableList with ContentId's to Content class and created hasChildren method.
Update ContentJcrMapper to set ids to new content field.
CMS-2013 Enrich content summary json with field: hasChildren

Updated ContentSummaryJson to put hasChildren boolean field to json.
Updated expected results for tests.
Updated RemoteContentModel.
Merge pull request #351 from enonic/CMS-2059

CMS-2059 Enrich Content with children