Add the ability for riders to sign up as backup-riders

[mveytsman]

In the spreadsheets riders can list themselves as backup riders, we don't currently support this in the rider portal.

We have a CampaignRiders table which associates riders with campaigns already - but in order to add a backup riders feature we'll probably need to add some sort of boolean flag to it to indicate that the user signed up as a backup rider.

There's also going to be an annoying state machine here If I assign myself to a task, then unassign myself I don't become a backup rider If I sign up as a backup rider first, then assign myself to a task, then unassign myself am I expecting to be back as a backup rider?

(My gut is there's a very simple way to model this and have the UX for it but I don't see it yet)

cc @sereprz and @teesloane who we were discussing this with just now.

[teesloane]

Considerations

Front End Work

• [ ] Signed up Backup riders should show up under nearth the right column in dispatch for campaigns under assigned riders.
• [x] Add UI for signing up for being a backup
• [ ] when unassigning yourself show a modal that prompts to ask if the rider wants to be a backup rider
• [ ] Ensure that signing up for a campaign, if you are a backup rider flips "is_backuprider" to false (prevent rider from being both a campaign rider and a backup rider)

Backend Work

• [x] add a "backup_rider" boolean flag on campaign_riders table
• [ ] When duplicating campaigns, should have a checkbox to duplicate previous backup riders

Nice to have someday

• being able to communicate how much capacity a backup rider has.