api: rethink user identity

[alejandromumo]

          This feels a bit out of place here. 

The problem was that to publish a record, we needed the user's identity. Therefore, we used a utility function from invenio-access to generate this identity given the user id.

But, there is permission needed to publish a draft (authenticated_user) and it was added here.

Possible alternatives:

1 - move this to invenio-rdm-records as this is a specific RDM action (publish)
2 - move this to invenio-access as a new utility function (e.g. get_authenticated_identity)

Originally posted by @alejandromumo in #99 (comment)