Build additions and fixes for build detail page

[agjohnson]

This adds a few missing features and fixes a couple UX and UI issues with the build detail page:

• Adds build retry button for PR builds, ported from the production dashboard
• Adds a build cancel button for active builds, also ported
• Tunes the JS observables
  • Drop computed for static observables, to reduce dependency chain and put updates in one place
  • Use observables for can_cancel/can_retry, instead of abstracting the data binding logic into templates
• Fixes a small display bug with loading icon
• Adds a helper CSS for hiding elements

Fixes #75 Fixes #145

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While running the cancel button will show:

After it disappears:

I can't easily give proof of the PR builder retry button, but it's in the same location.