Revert "Initial pass at Tumblr theme integration."

This reverts commit e1e406a587ec5291c2057c8c8e2d811919a2687c.
This was mistakenly merged to our `master` branch but should remain only
in the `tumblr` integration branch. My bad.
Remove Tumblr's `{CustomCSS}`. This should only be in `tumblr` branch.
Remove Tumblr's `{block:IfTweet1}`; should only be in `tumblr` branch.
Remove Tumblr's `<meta>` elements, should only be in `tumblr` branch.
Remove custom variable from `moment.js` call. Use actual string.