Fixed layout for collapsible table rows (was broken in Bootstrap 3.1.0).
Changed ID of main form for carpooling.
Improved and fixed carpooling controller and JS views.

- Factorized JS function is_error_field, checking if a given field is
erroneous.
- Now passing the passenger model properly to JS views.
- Changed passenger view to be more lightweight (ol / li).
- Changed table structure to be more lightweight: now only 1 tbody,
table striped alternates colours.
- Commented and DRY'ed out car_errors.js.erb
- Fixed and improved JS views for passenger (success and failure cases).