Resolve .hide/.hidden/.sr-only redundancy/confusion

Created by: cvrebert

.hide & .sr-only both have the same final effect. .hide & .hidden have confusingly similar names, but do slightly different things.

Visibility table:

Visual user	Screen reader	Class	Implementation summary
No	No	.hidden	// Hide from screenreaders and browsers
No	YES (usually)	.hide	display: none !important;
No	YES	.sr-only	// Only display content to screen readers