Opt-in setting to receive notifications when patches change status

https://bugzilla.yoctoproject.org/show_bug.cgi?id=7684

The way to opt-in would be to add oneself to the CC list of this series. A user subscribed to the CC list of a series would be alerted of activity.

A few notes:
- this needs to be a bit smart so we avoid flooding people with too many emails (ie don't send a mail per patch status change, but collapse several changes into one mail).
- is the patch submitter part of that Cc list by default?
- still need a global option, per-user, (and per project?), to disable notifications