Enlarge parallax background image

Avoid repeating the background.
Standardize on "let's take a look" before shellinabox iframe
Enable parallax background image
Revert "Enable parallax background image"

This reverts commit 9e6abc72043735d79cd03bf5027adb3c69910c86.
Add CC-BY-SA 3.0 license
Render links in bold print