Removes the auto_image/ directory

This is no longer needed, since we have generic templates now and no
longer generate colored icons.
Remove theme.css and theme-gen.css - outdated stylesheets
Remove siteisdown.html

We should potentially come up with a substitute that isn't site-specific,
maybe by rendering an error page to a certain location whenever the server
is started?
Merge pull request #1077 from learning-unlimited/remove_auto_image

Removes the auto_image/ directory