Summary: Adds attachment icon to collased message if message has attachment. I just copied the attachment icon in the thread-list to the message-list namespace in the static directory. If you want to do it another way, feel free to update
Test Plan: Tested locally. Not sure if this is worth a test, but I can write one if you want.
Reviewers: bengotow
Reviewed By: bengotow
Maniphest Tasks: T2085
Differential Revision: https://phab.nylas.com/D1687
