Allow admin to log in when review period is closed
Replace review period status with a closing date

It's currently not possible to set this via the admin interface.
Remove settings model and controller
Add controller for admin to update review period
Allow admin to open/close review period

The field for setting the closing date defaults to 30 days from now.
Expose review period closing time & time left
Generate & use a sensible closing time
Show time left on pages and in emails

This needs some design.
Report days left more precisely

Rails's distance_of_time_in_words is disagreeably imprecise above 28
days, and saying "about 1 month" for 30 days isn't helpful in this
application.

Note: dividing the number of seconds by (26 * 60 * 60) to get days isn't
exactly correct in every scenario (e.g. summer/winter time and leap
seconds), but that doesn't matter here.