Bootstrap's JavaScript modal system was interfering with our custom modals
because both used the .modal class. Renamed all modal-related classes to
use custom- prefix:
- .modal -> .custom-modal
- .modal-dialog -> .custom-modal-dialog
- .modal-header -> .custom-modal-header
- .modal-body -> .custom-modal-body
- .modal-footer -> .custom-modal-footer
- .modal-title -> .custom-modal-title

This prevents Bootstrap from trying to manage our custom modals.