Merge pull request #8918 from PeterLa/821388pla

Bug #821388 - Improve Reply, Reply-All, and Forward icons r=samjoch (cherry picked from commit 746b3fe9753f12de9a56a47b7de6502387ca3087)

Conflicts:
	shared/style/headers/images/icons/reply-all@2x.png
	shared/style/headers/images/icons/reply@2x.png