extracted html text helper methods into gem

test plan:
  * run specs

Change-Id: I8bd463396aee14853bdcf552eef144a509ffb62a
Reviewed-on: https://gerrit.instructure.com/29613
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>
QA-Review: Zach Pendleton <zachp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>