Miscellaneous fixes

Lottery preferences shouldn't apply to the day of the trips (on save,
will complain about trips that aren't open).

The `author` field was removed when feedback anonymity was removed.
Count attendance/flakes per trip, not per feedback

Assuming multiple feedback-supplying leaders per trip, the previous
scheme would unfairly affect attending and non-attending participants.
Change flake-scoring to only count once per trip with feedback.
Clarify distinction between admin/review trip

Add links, explanatory text to make distinction between the two, and add
some useful features.
  - Give review link on admin trip view
  - Only allow participant reviews once trip is over
  - Ask leaders not to delete participants if they were no-shows.
  - Show error if comments left out on form submission

Also leverages class-based views properly in both admin and review trip
views.
Add a simple "about" page