Proposal: Multiple speakers in submission

Background

I would have submitted this as a PR but first I want to run this by the maintainers as an issue, and if we can get agreement I am happy to take this on myself.

@paracycle and I were submitting a joint talk to a conference using this app but there was no clear way to indicate a second presenter. So I had to hack that information in. I think this is something that ought to be supported

Proposed solution

The backend already supports multiple speakers per proposal. But even developing locally I was unable to find a clear way to do this from admin.

I think not every conference would want to allow this, but perhaps there can be a boolean admin event setting for allow_copresenters?; and from there we have two possible ways I could see this be implemented:

Stripped information

We have one "main" speaker, but there are additional fields for copresenter emails. These are not stored as full fledged speaker objects but rather as an array of "copresenters" on the proposal object.

Full speakers

Use something like cocoon to allow the user to add n copresenters with all the first-level information of the main speaker.

For reviewers

Am I completely missing an obvious feature here that makes this issue moot? I think the admin could create custom fields for this but copresenting is relatively common so I reckon it should be a first class feature even as an option.

Review the existing admin flow and the proposal speaker support described in the issue, starting by confirming how multiple speakers are represented today and whether admin exposes that capability. Resolve the event-setting and copresenter-model choice with maintainers; done means an agreed implementation supports optional multiple presenters without affecting conferences that do not enable it.