Extend admin interface:

- Make admin controllers inherit from Admin::BaseController
- New admin dashboard with system statistics overview
- Centralized admin layout with dedicated navigation
- Enhanced user management interface with role assignment capabilities

RBAC fixes:

- Introduced new `admin` role that has all rights
- Added db migration to create admin role with appropriate permissions
- Updated admin rake tasks for role management
- Updated db seeds to add all required roles properly

Styling improvements and fixes:

- Upgraded button system with standardized styling and dark mode support
- Improved UI with consistent styling using Tailwind components
- Brought back manual dark mode using Tailwind custom variant

When adding a new conference, we now automatically run the import job and let the user choose relevant stages. Users can also manage relevant stages from the conference edit view.

Added tracking of import histories. The app now knows when an import has been started and ended, and errors are logged into the DB as well and exposed to the frontend.

Added an interstitial page for fetching conference data, plus a way to manually trigger imports if no data was found.