Add ability to define custom statuses

[engram-design]

I think it'd be neat to define your own additional statuses for various use-cases.

There will always be some default statuses that cannot be deleted (but could be disabled), which are opinionated about what they do:

• Approve
• Approve & Publish
• Reject

The UI will need to change to reflect the multiple options, so we'd do away with the 3 different buttons for actions. You'd now have a dropdown to pick the status and then submit.

Any custom statuses would then be up to you to define logic for in a module (checking against the status of the review), or you could leave it as-is, and the status becomes a visual representation.

We'll include a nominated colour with the status for the compare interface and other UI.

Related to #174 and #170