Global header: fixed position hides targets

[joyously]

When I click on a link to a comment, the browser loads the page and then moves that target to the top. But the header is in a fixed position at the top, so it covers the target.
I think it needs scroll-margin-top to account for the header height.
This example is using Firefox:

[ryelle]

This is a duplicate of #219 — closing this one even though it's older, since the other has some discussion.