R1:c3f95bc410a3

Add basic support for mail "stamps" to improve client mail routing

Summary:
Ref T10448. Currently, we use "mail tags" (in {nav Settings > Email Preferences}) to give users some ability to route mail. There are a number of major issues with this:

- It isn't modular and can't be extended by third-party applications.
- The UI is a giant mess of 5,000 individual settings.
- Settings don't map clearly to actual edits.
- A lot of stuff isn't covered by any setting.

This adds a new system, called…