In email, render dates with an explicit timezone offset

Summary:
Fixes T10633. When generating email about a transaction which adjusts a date, render the offset explicitly (like "UTC-7").

This makes it more clear in cases like this:

• mail is being sent to multiple users, and not necessarily using the viewer's settings;
• you get some mail while travelling and aren't sure which timezone setting it generated under.

Test Plan: Rendered in text mode, saw UTC offset.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10633

Differential Revision: https://secure.phabricator.com/D16287