Add the possibility to create collections

[cedricbonhomme]

A collection would be a named set of objects from a MOSP instance.

For example CASES would be able to create the collection named "Security measures for MONARC".
This collection can be dynamic:

• defined by one or several JSON schema(s). All objects instantiated with one of the JSON schemas will be part this collection.

And static:

• simply a list of cherry picked objects, from different schemas and organizations.

Also both static and dynamic.

Note: objects from a collection can be objects from different organizations.

Finally this will enable to share a set of related objects with a nice URL, like:

• http://objects.monarc.lu/collection/Security-referentials, or
• http://objects.monarc.lu/collection/UUID-of-the-collection

[cedricbonhomme]

Properties for a collection:

• UUID
• name
• description
• objects
• schemas
• creator_id