Issue subscription managment for all users

It's possible to manage your own mail subscriptions for issues via the subscribe button. There doesn't seem to be a way for an administrator to change other users subscription settings, or to change the defaults.

A useful feature would be to integrate this into the UI so administrators can change what all users, roles, or individual users are subscribed to.

(This comes from the default being None, and most users not bothering to change it, then claiming they didn't get emails about issues regarding them. There's only so many times you can remind them to change this setting themselves before you wish you could do it yourself).