KBFS backed email system: <user>@keybase.io

## Goal

Implement a KBFS backed way to receive email to keybase.io email addresses in the form of `@keybase.io`.
## Approach

> the emails would have to be written to a shared (private) folder between you and the keybase organization.
> - @strib
## Uses
- closes #2145 (Github signing/verification issue), closes #2181 (similar)
### History

This feature request was spawned from conversations in #2181.

Start by reading the related issues #2145 and #2181 and the proposed KBFS shared private-folder approach. Define how messages addressed as @keybase.io enter that folder and what receiving, privacy, and delivery behavior must be verified; the issue does not name implementation files or tests.

Written by the indexing model from the issue text.