Default replyTo field on std/email

[stevekrouse]

Right now the replyTo field is unset if you don't set it, but maybe it should default to something useful, ie the user's email address or the val that it was sent from's email address. However either of those are dubious defaults as well.

[stevekrouse]

We should probably let you set your default email handler, possibly in preferences or settings, kinda like how you point a custom domain at a val. Then stevekrouse.whatever@valtown.email (if whatever isn't a val on my account) or even stevekrouse@valtown.email would go to this default handler.