Fix leaving a proposal as an additional speaker.
Fix leaving a proposal as an additional speaker.

Since the `additional_speakers` field on the ProposalBase object uses
the AdditionalSpeaker object as a intermediary model, it is not possible
to "remove" speakers from this group.

https://docs.djangoproject.com/en/1.11/topics/db/models/#intermediary-manytomany

pydata/conf_site,pydata/conf_site,pydata/conf_site