WIP first draft of add co-speaker feature

[artcz]

No description provided.

[umgelurgel]

I'm not sure I understand the workflow; let's assume A creates a talk, B wants to be a co-speaker.

1. Should adding a co-speaker only be allowed during CfP?
2. B only sees his talks in user-panel - how should he access the preview proposal page for A's talk? Should the url be sent directly to him? If so, I guess we should add a button to A's preview that allows them to invite someone to become a co-speaker.
3. Right now, anyone with the reverse('preview_proposal', args=(talk.slug,)) url could in theory add themselves as a co-speaker. How should we permission it? Users could request to be co-speakers on any talk and the speaker would have to approve them, but that would lead to speaker spamming; or we could use the email mechanism from point 2, to ensure that only invited users can become speakers.

Also related to point 2 and cfp - should we disable submit_proposal_step2_add_speakers outside of cfp?

What do you think @artcz?