Roles management screens

[pierre]

New roles can be added today by going to /role_definitions/new (link hidden on the /admin_allowed_users/new form). It would be nice to be able to list and update them as well.

1. Add a ROLES link in the tag-select dropdown, below USERS
2. The link should lead to a page similar to /admin_allowed_users where roles are listed (new Kill Bill API needed)
3. Clicking on a role should present a form similar to /admin_allowed_users/X/edit where the role can be updated (the Kill Bill API already exists)

Suggested on the mailing-list here.