UI flow: have Collector generate collector credentials, provide them to Divvi Up

[branlwyd]

An idea from today's planning meeting: today, we will expect users to generate their collector credentials via a CLI tool. Another approach that would work would be to expect the Collector to generate & store these credentials, and use Divvi Up's API to communicate these credentials.

Some thoughts:

• Initial implementation could be by the already-existing API tokens -- this would work today, but requires the person configuring the Collector to provide this one token. An easier flow may be possible via OAuth, but this would require implementation work.
• Ideally, this would be something we could document well enough that non-DU-authored Collectors could use the flow, too.