Add support for multiple speakers to one proposal

[aridlehoover]

As a co-speaker, I want the ability to connect a proposal to multiple speakers, so that the same proposal doesn't appear twice when both speakers add it.

Given a visitor is viewing the new proposals page
When they click on the Speaker dropdown list
Then they are able to select multiple speakers

Given a visitor is viewing a proposal
Then the byline contains all of the speakers

[nodunayo]

Yes, brilliant, thanks!