Add dietary restrictions overview page to organizer console

This adds a page showing the totals per major dietary restriction (for now, vegan, vegetarian, gluten-free, and dairy-free) and the individual write-in restrictions, linked to from the organizer console.

A future improvement to this page should probably include how many people are RSVP'd per session, for ordering pizza / lunch accordingly. It's also possible that we should add some way for the organizer to see food preference by person on this page (perhaps a giant table like the attendee info page) in case they need to get in touch with people.

:pizza: :pizza: :beer: :beers: :pizza: :pizza: