Merge pull request #8475 from dominickuo/email-attach-from-gallery

Bug 838006 - Support attaching image files from gallery picker in email. r=asuth(cherry picked from commit 8311004db39832b7478b357be76e476944b3ba4b)

Conflicts:
	apps/email/js/compose-cards.js
	apps/email/style/compose-cards.css
	apps/gallery/js/gallery.js