Implement the activity profile resource

[lebrunthibault]

Feature Request

Implement the activity profile resource

Describe the solution you'd like
According to the spec : Stores, changes, fetches, or deletes the specified Profile document in the context of the specified Activity.
This means implementing CRUD access to the /xAPI/activity/profile endpoint as specified in the spec.
We would need to implement storage for all existing backends (the schema is free for this resource which is a document resource)

Discovery, Documentation, Adoption, Migration Strategy
Users will be able to access the new /xAPI/activity/profile endpoint as specified in the spec
One use case would be to store and retrieve activity metadata including a list of children activities (see #378)

Do you want to work on it through a Pull Request?
Yes I would like that