This makes three main changes:
* Adds a hook to transform download links.
* Adds a hook to add an externally created ActiveDoc to a DocManager.
* Rejiggers XLSX export code so it can be used without streaming,
which is currently tricky in a browser. Regular usage with node
continues to use streaming.
With these changes, I have a POC in hand that updates grist-static
to support downloading CSVs, XLSXs, and .grist files.
