Emoji's in email should be sized according to the surround text in css em's

This is a follow on from MDL-64598 and not sure if it's a bug or an improvement.

The current code assuming a font size 16px in the emails. Ideally we should be testing this in an email which has emoticons inside h1-h6 + paragraphs, even <small> tags etc. the emoji should be sized in em's relative to it's surrounding text not px.