Use our own instance of html_codesniffer

Our version will usually have newer updates and is the same as the one included in webaudit.unl.edu
Fix contast issues with carousel

The carousel controls were failing contrast after the update due to an rgba background.  It was comparing the solid text to the background, which was failing.  This is actually an invisible change, because the text is not actually visible.
Fix contrast issues with events

These are rgba related fixes
Fix contrast issues with tables

These are rgba related fixes
Merge pull request #770 from mfairchild365/tests-accessibility

Use html_codesniffer hosted by UNL for accessibility testing