Add status `CANCELED`

[nkaretnikov]

Fixes #547.

Description

This pull request: adds status CANCELED, which is displayed in the web UI when the user cancels a build.

Pull request checklist

• [x] Did you test this change locally?
• [ ] Did you update the documentation (if required)?
• [x] Did you add/update relevant tests for this change (if required)?

Additional information

How to test

• Run via docker
• Cancel a build via the admin UI by clicking on the cancel button
• The status of the build should say "CANCELED".

[netlify[bot]]

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