Add `rake backticks` task

This adds the Rakefile that I mentioned in #254. It contains a small script that checks for missing backticks across all pages. Now that all the missing backticks have been identified and corrected, I'm not sure there's much point, but here it is anyway.

You probably just want to close it immediately, but I thought it might be useful sometime in the future if a bunch more pages are added or something.