Fix editing close descriptions when a review request is closed.
The ReviewRequestEditor's editable field was recently changed to factor

in whether a user has permissions to edit the review request, and

whether the review request was open. This broke editing close

descriptions for closed review requests, since that depended on the

editable property as well.
Internally, Review Board has separate permissions for the abilities to

close review requests (and alter their close descriptions), and edit

review requests. That's now replicated in the JavaScript.
Instead of passing editable to the ReviewRequestEditor when

initializing, it passes mutableByUser and statusMutableByUser, and it

then calculates editable and statusEditable. The field editors can

respect either of these fields, depending on which field it is.

Tested with both open and closed review requests.
Unit tests pass.