Move most of custom header to meta partial

This puts the version comment stuff earlier in the file, so it's more
noticeable, and keeps the OpenGraph/Twitter card info near the other `meta`
tags.

Unfortunately, this does mean we need to copy in the original `meta` partial,
but it's only two lines.

Testing Done:
Checked rendered results and the meta tags were still there.

Reviewed at https://reviews.imfreedom.org/r/1929/