[SD-945] Improvements and missing files
feat(search): add search resource

search resource will be used for searching ingest/content/etc together.

SD-1057

Conflicts:
	superdesk/__init__.py
Merge remote-tracking branch 'siva/master'
feat(packages) - create packages

SD-923
SD-1036
SD-927
SD-1027
SD-925
SD-1039
feat(packages) - add tests for PATCH/DELETE.
feat(packages) - add two way link for associations
feat(packages) - check for circular references and duplicated links
feat(packages) - add support for groups/structure
when defining a package