Open Graph and Twitter Tags

Make sure that Facebook Open Graph and Twitter Card tags appear in each
slide's header so the proper information can be scraped by the social
network.
Fall back to presenter notes (truncated to 200 words) if the SEO description is not set.
Mobile optimization
Fix Funky BG Behavior

When the background color is not changed on a slide, it resets to the
presentation default when the slide is saved.  Instead, we should only
clear the slide color if it's changed to the same color as the default (to
allow "removing" individual slide customizations).
Make sure default bg appears as expected.
Modify footer behavior to match expectations
Resolve footer issues
Consistent color for navigation
Make navigation animate with the footer.
Don't close the footer when an overlay is present
Add overlay labels
Changelog