The Review Dialog's Add Comment button (used for general comments)

required a flag to be passed during construction. The "Review" action on

the review request passed this action in, but the "Edit Review" button

on the review banner did not.
Rather than fix up the review banner (plumbing the state through several

more objects), we now have a RB.EnabledFeatures dictionary where we can

set the enabled state for features we care about. The Review Dialog

accesses this directly, instead of taking in an option during

construction.
The dialog also no longer tries to load general comments if the feature

is disabled. This was causing an error before, given that the API would

not be accessible.
Unit tests were added to test both the enabled and disabled states.

Tested with the feature enabled and disabled. I only saw the Add Comment

dialog when the feature was enabled, and loading comments no longer

failed when disabled.
Unit tests pass with the feature enabled and disabled.