Show discarded review requests in the "All Review Requests" view.

For some reason, we chose to filter public discarded review requests out of the
list view. This made it very hard to find old changes that had been closed out
for some reason (they were only available in the submitter lists). This change
includes them along with "Submitted" review requests, and changes the show/hide
toggle to be "closed" instead of "submitted".

Testing done:
Discarded a review request. Saw that I could see it in the "All Review
Requests" view. Checked that "Hide closed" filtered out both submitted and
discarded review requests.

Fixes bug 576.

Reviewed at https://reviews.reviewboard.org/r/5303/