Flux source integration

[squaremo]

Addresses #158.

• [x] Design for Flux source integration
  • [x] Review that design and adapt
• [x] Implement!

[squaremo]

Further test cases:

• [x] When the referenced source is missing, the Stack fails and is marked retrying
• ~[ ] When the source has a new version, the stack is rerun~
• [x] If the source artifact URL cannot be fetched, the stack is failed and marked as retrying
• [x] If the Flux source is marked unready, this should be reflected in the Stack status; and once it's marked ready, the stack should be retried.

EDIT: I am going to bump "When the source has a new version, the stack is rerun". This is true, as a consequence of stacks being rescheduled, so it will work the same as a git source. It would be nice to keep a watch on sources and be more responsive than git sources (which poll the git repo for new commits on each run), but it can be done in a follow up PR, I think.

[squaremo]

(I will square away the design doc -- aside from anything else, the fields in the proposed API change need to be renamed)