Notifications when new milestones set

[r12a]

I haven't thought it out in fine detail, but it seems to me that it would be useful to send a notification email to those who want it when a change is made. From an i18n perspective, i'm particularly thinking about a new or changed wide review date.

Even if we do remember to regularly look for changes in the report, we may not necessarily spot them.

[madediosg]

I think this functionality will be really useful. Imagine you schedule an issue i1 that depends on another one i2 with milestone m1, so you schedule your issue according to that deadline.
Then, the milestone of i2 is changed from m1 to m2 so, unless you receive a notification of the milestone change, your issue i1 will be wrongly scheduled.

I think it would be very useful if milestone changes could be configured to be notified in subscribed issues.

Thanks!