option to send outgoing messages as PGP/mime html

[tomholub]

#1621

• This probably means we have to use PGP/MIME
• This also means we have to have a way for people to switch back to plain text. The UI should be similar to Gmail switch to plain text
• When someone switches to plain text, it will stay that way on their account, until switched back
• for now no actual html formatting. That's tracked as a separate issue

[tomholub]

#1861 is waiting for this

[tomholub]

duplicate of #1620