Implements Simple Admin

Implementation of the [Simple Admin](https://trello.com/c/NAtggRqE/1224-simple-admin-for-registry) story.

* Users can be configured as administrators and have admin powers over all packages
* Non-admin users can admin their own packages
* Delete a package (metadata only - package files are still in S3)
* Change ownership (so another user can start uploading)
* Change requirements (can prevent Extension Manager from downloading packages... does not actually change the package file itself, so you can't relax the requirements with this feature)