unoconv is deprecated / better solution for isolating LibreOffice

[ganwell]

In theory we could switch to unoserver but it has the same issues as unoconv.

To fix the problem of hanging LibreOffices and race-conditions every call to LibreOffice should use --env:UserInstallation=$RANDOM_UNIQUE_TEMP_DIR. Of course it is not 100% certain that this works, until we tested it.

Using unshare to isolate LibreOffices needs the settings:

    cap_add:
      - CAP_SYS_ADMIN
    security_opt:
      - apparmor:unconfined