Support subpaths/projects per stack

[DazWorrall]

Allow stacks to target a subdirectory of the repo. This needs a lot more testing and is not ready, for now just seeing if it looks feasible.

Things that would definitely need to be fixed:

• Reading shipit.<env>.yaml would need to be subpath aware
• Task execution would need to happen with the correct working directory
• Consider impact on stack uniqueness
  • Can a repo have multiple stacks with different paths each with their own production env?
  • How should paths be presented in the UI, particularly the stack list

[wayt]

This could be a great addition to work in pair with https://github.com/Shopify/infrastructure/pull/1266

[wbean1]

💯 on this idea. would be very beneficial to cluster-service-ingress-nginx in Shopify/infrastructure, and I'm sure there are more cluster services in similar situations.