ManualPermissions can now be created, modified, or deleted through the admin

site. I decided to go with a raw ID field for user since a drop-down isn't

the best for Review Board instances with many users, but we can change this in

the future if we'd like to.

Added, modified, and deleted ManualPermissions successfully.
Tried entering invalid IDs for the user and local site, and got the expected

  error messages.