Deleting a branch on GitHub triggers a build

[humitos]

When a branch is defined as an active version on Read the Docs, deleting it from GitHub will trigger a build on Read the Docs which will fail because when trying to pull down that branch it will fail.

The webhook received on Read the Docs is a PUSH event with a particular field deleted: true. So, we may want to detect these and skip triggering a build for those. Also, we should double check that the version for that branch is deleted.