webstaff: UI for various serial notes

webstaff: UI for various serial notes

These changes add menu options for Subscription Notes, Distribution Notes, and
Item Notes to the webstaff serials interface (under Manage Subscriptions and
Manage Issues).  They spawn dialogs similar to the existing Copy Notes dialog
from the Item Editor, and show and allow the editing of existing notes as well
as the creation of a new note.

I'm not attempting to implement any behavior involving the alert flag.  I don't
know whether that exists or not.

Signed-off-by: Jason Etheridge <jason@EquinoxInitiative.org>