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.

Cf. 38931205

Also account for absolute/relative paths

We don't want the "canceled stage" to be filled with talks we've never
even seen before.

When talks are removed from Fahrplan, move them to a dummy stage.

Cf. https://github.com/pretalx/pretalx/issues/1910