robustify Differential and Maniphest mailhandlers wrt attachments

Summary:
a few things

• make the parent mailhandler class not send "blank body" error if you have attachments
• make both differential and maniphest append a list of attachments to the body if any exist
• BONUS - made the cc stuff work in Maniphest

Test Plan: I haven't actually tested this yet. :( i need to figure out how to send a mail with an attachment from the command-line and figured I'd serve this up first.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2012

Differential Revision: https://secure.phabricator.com/D3868