attempt correcting display of overlapping events

Closes #246 and #243

See merge request degrowth/scriptorium!22

This is how it should have been done in the first place. An additional
condition proves enough to guard against unscheduled events, that do not
even have time slots defined.

This reverts commit cf476386.

Implement new session types and update another

Closes #241

See merge request !21

Implement working schedule and cosmetic changes, fixes #239

Closes #239 and #237

See merge request !20

Implement new session types, fixes #240

Closes #240

See merge request !19

Issue-232 integrate pretix-widget

Closes #232

See merge request !18