Add the "Add billing diagnosis" privilege on the user roles page

[mnorbeck]

#893 Expected behavior:

Should be able to grant and revoke that privilege on a role.

Actual behavior:

privilege is not seen on the page.

Steps to reproduce:

1. Nav to the user roles page, select any role.
2. Privilege is not seen.

Screenshots (if applicable):

OS and Browser:
OSX 10.10.5, Chrome 55.0.2883.75 (64-bit)

@jkleinsc