It is now possible to extract email bodies from nested multipart messages

This fixes an issue where receiving a nested multipart message would
show some of the raw message pieces instead of the actual message. A
new test case has been added.

Fixes CNVS-5873

Test Plan:

- Set Notifications for Conversations so that you will be emailed to an email account you have set up in Outlook.
- As another user, message that student to prompt the notification to be sent
- From Outlook, reply to the Conversation Message notification and include an image somewhere in the body of your reply
- Return to the Conversations Inbox of the user that was being replied to and observe that the message displays in Canvas with all the encoding data.

Change-Id: I497cf421ee231d7e95c0bacd34cb2448fb7220ff
Reviewed-on: https://gerrit.instructure.com/38684
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>