Allow multiple Speakers for a session

[jandd]

Is your feature request related to a problem? Please describe.
The current data model does only allow a single PublishedSpeaker/Speaker for each Talk instance. There are talks/sessions that are performed by multiple speakers and we should allow this in the data model.

Describe the solution you'd like
Allow adding additional speakers for a talk. The new speaker should have its own speaker data and should be able to edit the data by him/herself.

Additional context
Data model, export scripts, frontend and test data and tests have to be adapted.

[jandd]

I started work on this feature in a separate feature branch at https://github.com/jandd/devday_website/tree/multi-speaker-support

[jandd]

Multiple speakers are supported in the backend. There is no support to add another speaker for speakers during the CfP period yet.