Detect whether a page is a snapshot by reading its <meta> and <link> tags.

[Treora]

Moving this here from issue #138:

We could still show info about the snapshot (it’s date and original URL): when a locally stored snapshot file is opened in the browser (with file: URL), the extension could read the DOM and detect the & tags that were added by freeze-dry, and thus recognise it as a snapshot. I am not sure however which browsers load and which refuse to load extensions in pages with file: URLs, but at least Firefox appears to load them nowadays.