Deployment dependencies: UI surface

[adityachoudhari26]

Sub-issue of #1067.

Surface deployment dependencies in the web UI:

• Show a deployment's declared dependencies (and their version selectors) on the deployment page, with the ability to add/edit/remove.
• On a release target / release view, indicate when a release is blocked by an unsatisfied deployment dependency, including which dependency and selector failed.

Depends on the API wiring (#1078) and benefits from the re-trigger plumbing (#1079) being in place.