Proposal has_many :speakers

[esmale]

What does this PR do?

While the norm tends to be for a proposal to have a single speaker, there are also sometimes panel discussions or presentations involving 2-4 presenters. As such we want to support associating multiple speakers to a single proposal.

Why was this work done? Is there a related Issue?

Add support for multiple speakers to one proposal

Where should a reviewer start?

Are there any manual testing steps?

There's one area of visual concern that I'd appreciate some feedback on. The new/edit proposal form allows you to select multiple speakers from the dropdown, but visually it's not very clear. I wasn't able to find anything in the documentation of getskeleton to provide a good visual for multiple-select, so not terribly certain what to do here.

There's also the additional wrinkle that my other (slightly infamous) PR, Add passwordless for authentication, removes the ability to specify a speaker under the assumption it will be the current user. So I'll need to revisit that to bring back the speaker selection, but ensure that the current user/speaker is always included in the list.

Screenshots

N/A

Deployment instructions

In addition to the database migration to create the proposal_speakers table, I added a rake task (migrate:speakers) to map the current speaker-proposal data into the new join table. That rake task would need to be run after the database migration is completed in order to retain the current mapping between Proposals and Speakers

At some future point after this PR is merged, there could be a follow-up PR to remove the speaker_id column from the proposals table.

Database changes

There's a new database table, proposal_speakers, which serves as the join table between the Proposal and Speaker models to allow for an arbitrary number of speakers on a single Proposal record.

New ENV variables

N/A

[nodunayo]

@esmale As discussed today, all sounds good, but we'll focus on #586 first. :-D