Fix allowing admins to reopen discarded review requests.

Due to a bad conditional, it wasn't possible for an admin (or user with
the appropriate permissions) to reopen a discarded review request. This
worked for submitted review requests, but not discarded.

The conditional now includes the permission check.

Reviewed at https://reviews.reviewboard.org/r/6144/
Merge branch 'release-1.7.x' into release-2.0.x