Header bar cleanup.
Major changes to checklist.

- Abstract checklist items with ChecklistItem model.
- Component-ize the checklist item rendering.
- Make entire list group item box a hit target for marking as complete.
Shorten make ready for Zipcar button text.

Was clipping outside bounds on narrow mobile devices.
Clarify naming.
Fix move to Zipcar fleet.
Extract UserAwareController mixin.