Allow reviewers to always be able to trigger the automated review of snap revisions

[Alex Murray]

Currently, reviewers are able to trigger automated review for some snap revisions and not others, and it is not obvious why this is not always possible. It would seem to be that once a snap revision has been manually rejected, that it is not possible for a reviewer to ever be able to trigger the automated review for that revision, unless the uploader requests a review. This makes the process to approve snaps (once say updating the snap declaration) a lot more tedious than it should be - if instead there was always the option to re-run the automated review this would allow reviewers to drive the entire process without needing help from the snap uploader.