Add Parsoid CSS module

Parsoid's HTML needs additional CSS to render images, references,
external links, etc. correctly.  Define an 'ext.parsoid.styles'
ResourceLoader module for these for use by Parsoid clients (VE and Flow
pages).

This first cut adds the CSS to style <span class="reference"> to
resemble <sup>.

Bug: 51245
Bug: 60017

Change-Id: Id964b6eb210422ccf78639d2e64dd9140fa37e15
Merge "Add Parsoid CSS module"