Add feedback about status of mixnet in Pixelated UI

Definition of done

• an icon changes status (grey/green) in the compose window if the message is going to be mixed
• an icon changes status (green) in the email view window if the message has been mixed

Steps

• write requirements for the client side (bitmask)
• design server API spec
• add mixnet component to core
• hack on pixelated widgets
• parse mixnet-service.json
• switch delivery to smtp proxy for mixnet

Specs

• a composing message will be mixed if: a) the recipient provider belongs to the mixable set (advertised by our provider), b) the user has opted-in to mixing (in bitmaskd.conf for now), c) the mixnet is healthy (query status api in the platform).