Change all full screen images to slide background

This way, we can use the CSS "background-size cover" and
"background-size contain", which makes the images display nicely
regardless of viewport size or aspect ratio.
Switch QR code background to fully-opaque white
Use em value, not percentage, for iframe height

Chrome/Chromium and Firefox differ in their interpretation of the "width"
property, set in CSS, on an iframe. When given a percentage for "width",
Chrome interprets this as a percentage of the surrounding container,
whereas Firefox seems to take a percentage of the height of the document
displayed *inside* the iframe.

To work around this, give the height in ems, not in percent, using
a reasonable-looking value of 15em.
Reduce maximum image size to 75% of screen

Ensure that QR codes properly fit on the screen.
Full-screen CC-BY-SA logo
Improve summary slide notes
Add missing whitespace

Missing newline would cause Horizon slide to be displayed as notes to
previous slide.
Merge branch 'master' into gh-pages