backend: add cancel update buttons

[landism]

1. For each resource created by a Tiltfile, the Tiltfile reconciler adds a UIButton to cancel builds for that resource.
2. The build controller watches that button and cancels builds as appropriate.

This is hidden behind a feature flag pending a frontend PR for styling. Until that's done, this works fine and displays as a normal, user-added UIButton in the web ui. We could probably just ship with a bit of icon/text tweaking, but the frontend work should be pretty quick.

Notes

• I'm kind of on the fence between a) one cancel button per manifest (like in this PR), b) one cancel button per build (have the build controller create the UIButton when starting a build and remove it when it's done). The latter hadn't occurred to me until I'd already implemented the former, and I haven't come up with any super compelling arguments either way. I think it'd be pretty easy to change, if anyone wants to make the case.
• We'll probably want a CancelOnSpec at some point, but I'm not sure it's worth it before we put this stuff in the API.

[shortcut-integration]

This pull request has been linked to Shortcut Story #13230: implement cancel builds on backend.

[milas]

RE: Manifest vs per-build, your approach makes sense to me. I'm struggling to think of a valid use case for canceling an individual part of the "build pipeline" for a manifest, so not sure if there'd be a tangible benefit to that. (Also, from a purely UX perspective, having buttons popping in and out seems icky)