feature request: switch from PGP to Saltpack in web

Hereby would propose to use Saltpack in web when offering encryption, decryption - just like in app. Currently when non-Keybase user sending message via web, it comes in PGP format that cannot be decrypted in web when not hosting private key ([considered not secure](https://github.com/keybase/keybase-issues/issues/1127)). Especially in case of smart device there is no PGP key usually available even in command-line but only Keybase. So, using Saltpack format for encryption could allow decrypt then regardless what to use. As you preach also at https://keybase.io/blog/crypto - _Saltpack is a general format. We'd love to see its wide adoption_ - so, please, adopt it!

Review the web encryption and decryption flow, starting with the security concern linked in issue #1127. Done means web messages use Saltpack rather than PGP and support decryption in the stated non-Keybase and private-key-unavailable scenario; compatibility and implementation scope need clarification first.

Written by the indexing model from the issue text.