Improve snapshot integrity

[annevk]

Apart from #97 a problem we have with Commit Snapshots and Review Drafts of HTML (maybe this is also true for Encoding and others?) is that they point to resources which may be removed in the future.

What we should be doing is duplicating the resources (and dedupe them on the server with the script Philip found every now and then) so each snapshot is self-contained.

The main problem with this is that we have to rewrite the links from the multipage documents to go one directory up. Such functionality probably existed in wattsi in the past though and it might not be that hard too restore. We'd have to look at the reason again why it was removed (or perhaps it was just thought to be a simplification since we didn't need it).

[annevk]

(This is a follow-up to #151.)