This commit fixes 3 small issues with the way we produce our Qubes RPM:
1. The `.exists()` method follows symlinks by default, whereas we want
to check if a symlink exists. This functionality has been added in
Python
3.12.
Instead of checking if a symlink exists and then removing it, simply
remove it and don't throw an error if it doesn't exist in the first
place.
2. The `dz.Convert*` policies were not installed with the executable bit
set, therefore the qube could not start.
3. The `dz.ConvertDev` policy in particular had an ambiguous shebang,
thus we change it to explicitly call Python3
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.
