R1:afae26ad941a

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…