Update content.json under Composing section

Added Hack the Notebook Day teaching kit, and fixed name on Teach Intro HTML kit - both changes within Composing section.
Removed the link to gallery search
Merge pull request #850 from brettgaylor/fixgallerysearch

Removed the link to gallery search
Merge pull request #848 from keyboardkat/patch-5

Update content.json under Composing section
Fixes Bug 1027273 - Add embedded videos to Privacy resources page Proof of Concept
Fixes Bug 1031582 - Add embedded videos to Privacy and Navigation resource pages