Email diff notifications per user (BB-7394)

It would be nice to be able to control email/email diff notices at the user level instead of at the project level.

ex:

UserA has write (non-admin) access to ProjectA, ProjectB, and ProjectC.
UserA wants to get Email Diff notices for ProjectA and ProjectC, but not ProjectB without involving the various project admins to add UserA's email address to the Email Diff hook.