CRM-20798 Meet WCAG 2.0 AA accessibility standard

This is an epic for issues involved in improving CiviCRM accessibility so that it meets WCAG 2.0 (https://www.w3.org/WAI/WCAG20/quickref/). We should likely prioritise A, then AA, and finally the AAA levels.

Compliance with this standard would improve the usability of CiviCRM for important segments of users. It is also required by law for government websites in a number of jurisidictions (US, Canada, Australia), and in some places for all websites of organizations in a jurisdication (eg the Canadian province of Ontario, Australia).

In Australia, WCAG compliance is required by Australian Law and applies to all websites not just Government. See excerpt:
"This requirement applies to any individual or organisation developing a website or other web resource in Australia, or placing or maintaining a web resource on an Australian server."
From https://www.humanrights.gov.au/world-wide-web-access-disability-discrimination-act-advisory-notes-ver-41-2014#required
 
An obvious example of this would be a non-profit that uses CiviCRM and that has users with visual disabilities. Their website must be WCAG compliant to the required level. No exceptions.