container: Run LibreOffice in safe mode

The main use of safe mode [1] in LibreOffice is to run with a fresh user
profile, in case the default one got borked somehow. This is actually
not a concern of ours, since the user's profile is in the container and
is not persistent.

The main reason we want to preemptively run LibreOffice in safe mode is
to remove hardware acceleration capabilities. Whether hardware
acceleration actually works in a container is another question, but we
want to be extra sure.

[1]: https://help.libreoffice.org/latest/en-US/text/shared/01/profile_safe_mode.html

container/dangerzone.py

@@ -238,6 +238,7 @@ class DangerzoneConverter:
             args = [
                 "libreoffice",
                 "--headless",
+                "--safe-mode",
                 "--convert-to",
                 "pdf",
                 "--outdir",