offer static schedule export in orga area & API

[luto]

As of #269 there is a management command to export for all public schedule pages as static HTML to data/htmlexport (or a different path, as configured). To make this accessible to non server admins, there should be a button to generate an export on-demand in the orga area.

• add a celery task to generate schedule in background
• add button in orga area to trigger celery task
• add optional zipping to exporter
• offer generated zips for download in orga area
• integrate into the readonly API