Add heading anchors for h2 through h6.

[bryanbraun]

Fixes peterdesmet/petridish#7

This displays the anchors to the left of the headings, on hover, and uses your default link styles.

It won't show on .card-title headings or the headings on the team page, but it should show for all other headings from h2 to h6.

To test it out:

1. Pull down the branch and run npm run copy:all
2. Start the server, browse to /about, and hover over the headings to see how it looks.

Let me know if you want to adjust anything. 😄