Link to email apps when appropriate (not just webmail)

[jason-curtis]

Lots of folks use apps for their email these days, rather than webmail. Support this somehow.

Basic possibilities:

1. Add IOSAppUri and AndroidAppUri to our existing API response, and have the caller figure out which one to use. Downside: more complex for caller.
2. Rather than just a GET endpoint, implement a /redirectToEmail/?email=X endpoint that runs some JS and then redirects the user to the appropriate place (be it webmail or native app). We would need to handle the failure case, maybe with a landing page that links to a variety of email providers. Advantage: Super simple for developers to integrate with. Disadvantages: probably slower, plus developers will probably be less excited about redirecting users directly to us.