notify trackers via email on identity changes

To experiment with tracking, I set up two keybase accounts with different email addresses. Each account tracks the other, and I was hoping that one account could be used to audit the other in case one of the email or keybase accounts was compromised.

On the invitations/notificaitons screen (https://keybase.io/_/user/account#link-tab-invitations), it says:

> Receive emails:
> when someone tracks me publicly
> when someone I track changes their key or identity
> important news, once in a great while

To test out the identity-changes notification, I revoked and replaced the Gist proof for one account; however, I did not receive email notification in my other account saying the tracked account had changed.

Start at the invitations/notifications screen linked in the report and reproduce the identity-change scenario with two accounts: revoke and replace a Gist proof on one account while tracking it from the other. Trace the email notification flow for tracked identity changes; done means the tracking account receives the expected email.

Written by the indexing model from the issue text.