Bug 1087015: Demo Stuido - gallery page

- Gallery page now stacks
- Replaced JS that created "card" effect on hover with pure CSS
- Revived background image on not-the-landing-page
- Restricted button styles to act within main content
- Added clearing to all columns when they collapse.
Merge pull request #2853 from stephaniehobson/BUG-1087015-demos-gallery

Bug 1087015: Demo Stuido - gallery page