Registrasion updates

Show applied vouchers on attendee registration admin page.
Add head title and page title to more areas.
Update the dashboard to show warning for empty categories and have
button to open voucher page.
Fix exception on attendee page when user does not have attendee object.

Update mobile schedule

Display track details on talks if available.
Show day end time slot as per desktop view.

Styling for schedule

Update to Bootstrap 4 throughout.
Remove unused blocks and elements from past conferences.
Ensure schedule works in print mode.

Fix price filter

Label or help text could be unassigned, so convert to empty string.

Finalise registration

Add shirt types and sizes. Improve messaging about discounts.
Restyle ticket wizard and product category screens. Enable page titles
and messages. Update dashboard to hide raffle.
Enable inventory population for dev container.