Fix bug in review_section

/2014/reviews/section/talks/ and similar pages were not showing all the
proposals they should have, due to a bug in the review_section view.

Also changed the review_assignments view to grant access to all superusers,
for consistency with other views.

Also added some doc about how to enable proposal submission, and added a
couple help text strings for the date fields that control the dates
submissions are open, and fixed a bunch of PEP-8 problems.