Dev: Send emails when someone is added / removed as a domain manager

As a domain manager I want to know when managers have been added/removed from my domains so that I'm aware who has access to those domains.

[ ] Email is sent to all managers of a domain when someone is invited to manage the domain.
- [ ] This email should be sent whether person was invited via the registrar OR via the admin.
[ ] Email is sent to all managers of a domain when someone is removed as manager of a domain.
- [ ] This email should be sent whether person was removed via the registrar OR via the admin.
[ ] Update the existing email that is sent to the person who has been invited to a domain. Text updates are highlighted in green within this document.

CONFIRM DURING REFINEMENT:

Do we want to send this even when initiated in the admin?
For Org Model, someone could be added to (or removed from) dozens of domains via the "Members" page. Is it ok that dozens of emails could be sent to the same people?

cisagov / manage.get.gov