Added help text to event model.
Corrected form field error HTML markup.
Added special-case handling of checkbox inputs.
Added basic cart view and cart count summary.
Implemented initial housing view.
Restrict 'nights' fields to event housing dates.
Added basic 'save as defaults' behavior to housing form.
Added workaround migration for django bug #22432.
See https://code.djangoproject.com/ticket/22432