Move h's reply and mention notification emails to a sub-account

[seanh]

Reply notification and mention notification emails are sent in response to user actions (creating replies or annotations with @mentions in them) and could be abused, for example to send spam.

It would be bad if a burst in these types of email filled up our Mandrill quota and prevented more important emails from being sent (for example account activations, password resets, ...).

To address this create a new "sub-account" in Mandrill, change h to use this sub-account for reply and mention notifications, and add an hourly quota to the sub-account.

See also:

• https://github.com/orgs/hypothesis/projects/153/views/1?pane=issue&itemId=102470083 (the same issue, but for mention notification emails sent by LMS)
• https://github.com/orgs/hypothesis/projects/153/views/1?pane=issue&itemId=102896489&issue=hypothesis%7Ch%7C9411 (implement per-sender email limits in application logic, which is a separate issue)