Add rel=edit link to edit this page link

[capjamesg]

This PR adds the rel="edit" attribute to the "Edit this page" link in the sidebar of pages in the theme.

rel=edit lets a page indicate that the linked resource can be used to edit the page. It is defined at https://microformats.org/wiki/rel-edit. This can then be parsed by tools like the Universal Edit Button and custom bookmarklets to open the edit page corresponding with a website.

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