R1:1b24b486f58a

Manage object mailKeys automatically in Mail instead of storing them on objects

Summary:
Ref T13065. `mailKey`s are a private secret for each object. In some mail configurations, they help us ensure that inbound mail is authentic: when we send you mail, the "Reply-To" is "T123+456+abcdef".

- The `T123` is the object you're actually replying to.
- The `456` is your user ID.
- The `abcdef` is a hash of your user account with the `mailKey`.

Knowing this hash effectively proves that Phabricator has…