[MlMt] JMAP support

[davecc0000]

> On Jan 11, 2019, at 12:10 PM, Sam Hathaway <list.mailmate at munkynet.org> wrote:
> 
> If coordinated right, it would centralize the thankless work of kludging around nonstandard, broken IMAP server implementations. Imagine if, instead of each MUA author having to develop, test, and maintain dozens of ugly hacks, they were collaborating to improve a single JMAP proxy codebase that all could use.

Puts 1 more level of responsibility in the chain. If users have mail issues, no matter how responsive and responsible the client developer is, they can only say “We’ve contacted the proxy developer. You’ll just have to wait.”

Dave