Allow setting favicon for site, document

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We should be able to set the favicon for the page from the site and from each individual document.

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The Site object needs images, just like the documents.

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We're just going to set this on the site for now. If someone wants it on a per-document basis, they can add it later.

We should likely do that by making an images helper, which goes from the document, to the app, to the site, to find the right list of images. This idea might also work for a links helper...