Update content.json

Added image and gif as first sample bits of media for the page.
Update content.json

Trying to make suggested edits (altho I wasn't sure about where the last suggestion was in code. Is this right?)
Update content.json

Line 656 fixed (I hope!)
Merge pull request #904 from keyboardkat/patch-17

Fixes Bug 1040953 - Add media [images, videos] to "Composing" page