Add two RPC calls that can run on disposable VMs:
* dz.Convert: This call simply imports the dangerzone package and runs
the Qubes wrapper for the "document to pixels" code. This call is
similar to the way we run the conversion part in a container.
* dz.ConvertDev: This call is for development purposes, and does the
following:
- First it receives the `dangerzone.conversion` module as Python
zipfile. This way, we can quickly iterate on changes on the
server-side part of Qubes, without altering the templates.
- Second, it calls the Qubes wrapper for the "document to pixels"
code, as dz.Convert does.
