The diffs shown for text fields on a review request were leaving Emojis

at their native size of 64x64, which is too big. That was due to the

width being overridden to auto for images in those fields.
This change fixes that by adding an explicit height for Emoji images in

the base rule, and allowing the width to remain auto.
An &:extend LessCSS rule was also removed, as it wasn't really needed.

Checked that Emojis were looking correct in review request fields and in

the diffed content.