Adding the Message::isForwarded property

[chibenwa]

Hello,

At Linagora, we tend to consider forward information as important
for the email we care about.

Today, it is not part of the RFC-3501 spec, and many IMAP
implementations handle it with the de-facto standard $Forwarded flag.

This implicit standard is a bad thing, and we truly would like the JMAP
mail protocol to do this right. To be right, it should be explicit.

We then propose this pull request.

It reproduces the behaviour of answered feature:

• Adds a isForwarded message property
• Adds a mechanism for automatically marking messages as forwarded upon
  sending emails
• Clarifies interactions between isForwarded and threads
• Makes isForwarded searchable

Does this proposal make sense to you?

[neilj]

As discussed on the IETF JMAP mailing list, we're going to provide generic support for IMAP keywords rather than adding another explicit property. I'll push a pull request with this soon.