fix: Ensure JS event listeners use turbo:load

Replaces DOMContentLoaded with turbo:load in several views
to ensure JavaScript behaviors (like toggles, modals, and tabs)
are correctly initialized after Turbo Drive navigations.

This addresses issues where UI elements would not function
correctly after a Turbo navigation until a full page reload.

Affected files:
- app/views/devise/registrations/edit.html.erb
- app/views/admin/conferences/import_history.html.erb
- app/views/shared/_admin_nav.html.erb
- app/views/layouts/application.html.erb
- app/views/admin/conferences/import_progress.html.erb
- app/views/assignments/by_user.html.erb

Cleaned up redundant listeners in application.html.erb and
_admin_nav.html.erb.