Fix plugin 404s.

I noticed that of the seven custom plugins listed in ckeditor-utils.js,
only two were listed in build-config.js and the other five were the ones
that 404ed on prod.

I also added the scratchpad icon to source control. I had to add it by
`git add -f` since it's ignored, but I have no idea from where. Not
listed in .gitignore. I also noticed .git is a file pointing elsewhere.
It's all very mysterious.

Test plan:
Load "A beginner's guide to the history of western culture" in the
editor - see there are no 404s (not that there were before).

Auditors: tom