Add platform independent file opening

[redshiftzero]

We use QProcess to open files in a disposable VM using qvm-open-in-vm, which is a Qubes-only program. We could use xdg-open to open files such that this file open process would also work in Linux, but by always using qvm-open-in-vm, we get defense in depth as we will not accidentally open a file in the sd-app AppVM if there is a misconfiguration with the mime type handlers in the sd-app VM.

The problem with this is that now on Mac or Linux, we can't open files. We can apply a patch, like:

diff --git a/securedrop_client/logic.py b/securedrop_client/logic.py
index 8b5c27c..99bbe4f 100644
@@ -462,6 +462,11 @@ class Client(QObject):
             process = QProcess(self)
             process.start(command, args)
         else:  # pragma: no cover
+            # Running on mac.
+            command = "open"
+            args = [submission_filepath]
+            process = QProcess(self)
+            process.start(command, args)
             # Non Qubes OS. Just log the event for now.
             logger.info('Opening file "{}".'.format(submission_filepath))

for testing, but this is not convenient for developers. Another approach is in the above diff, to subprocess out to a script that calls open (macOS) or xdg-open, depending on what the current platform is. This ticket is for implementing that approach (or a better idea if you have one!).

(Followup to #113)

[eloquence]

This is very nice to have for development but not absolutely critical for beta since we must test everything in Qubes anyway, so added to Post-Beta milestone for now.

[eloquence]

From 2022-08-22 review with @nathandyer @cfm @gonzalo-bulnes @eloquence:
In practice, since opening this issue, we've been doing all our testing of opening files in disposable VMs in Qubes, so this has not re-emerged as an urgent need. Closing for now, we can revisit if it turns out to be useful e.g. for CI or other automation.

[gonzalo-bulnes]

Loosely related to #1541, which should make swapping QVM for something else easier.