[RBAC] Add something to show general global capabilities for a user

[AlanCoding]

Take the AWX /api/v2/me/ endpoint. This proposes to show a dictionary of all the permissions that user has in the abstract.

This would include or not include "awx.add_inventory" based on whether the user has any general capability to add an inventory to any organization.

This information would be used by the UI to show or hide the "ADD" button in the inventory view.

[AlanCoding]

Unclear where this data would be surfaced, so not apply the read-to-work label.