Feature: Approve/Needs Work/Decline Review

[ty1824]

A user should be able to approve, decline/reopen, or request additional work on a review using the plugin.

This should be exposed as a trio (+/-) of buttons in the top-right-hand corner of the Active Review View (on the same line as the branch status).

When clicking one of the options, a popup should show giving the user a chance to enter an approval message.

Some hosts may not allow certain actions, this should be configurable via a new method on ReviewHostDefinition.

Discussion:
Should decline/reopen be exposed on the top-level UI, or should it be hidden inside a "additional options" dropdown? Bitbucket takes this approach - declining affects the overall state of the PR, not just what states it can transition to.