Now that the code is public, having a hardcoded string seems even less
prudent...

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.

This partly reverts 658ea7d7.

The configuration was added by solid_queue:install but we are using the
same database for storage and queue.

This includes a two-part update of solid_queue via 0.6.0 as requested in the changelog.

Login still works regardless of case!

Required by the more languages team.

Don't have the old secret. Apparently nothing of importance in them.

This allows signup, profile edits and login/logout.
Note that due to poor naming, there are conflicts between our session
model und what devise thinks a session should be. In particular, we have
to rename `new_session_path(resource_name)` -> `new_user_session_path`
and `session_path(resource_name)` -> `user_session_path`.

Cf. https://github.com/heartcombo/devise/issues/2800

The state is now fully client-side and handled in session_controller.
This allows us to continue to broadcast the session frame to everyone.