Files with embedded resources

[btrask]

To support files with content embedded from other files, we need to do a few things:

• The preview generator needs to resolve the hash URI (which currently it does incorrectly...)
• The meta-file synthesizer needs to recognize the embedded URI and record it in the meta-file
• The sync algorithm needs to track and resolve these dependencies for each file, ensuring that the dependencies are added before the meta-file that declares them (for consistency)

We should block dynamic/external embedded content. We need a list of acceptable URL schemes (hash: and data:, at least).