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.