Revert "Add permalinks to post headers"

Dropping the approach of inserting permalinks at the generation stage. Jekyll
just doesn't offer fine grained control over which pages will receive these
additional elements and which wont.

Going to try a different tactic inspired by Jekyll's own website: Insert a JS
snippet into the `posts` template that dynamically adds the permalinks to
header elements.

This reverts part of commit a9a7d4fd9d16059066fcf5c8249f124824671d38.
Use JavaScript to generate header permalinks

Using this approach, we use the built-in ability of Redcarpet to tag header
elements with id values. Then we add a bit of JavaScript to the template for
posts that adds permalinks pointing at these ids.

TODO: The typographer in me wants these links to hang in the margin. Figure out
how to do that with CSS.