Cleans up "default" option for mimetypes in config pkg

Pointed out by @emkll during review, the ".install" declarations use
directories as dest, so we can't write a file that way. Added to
postinst, along with the other setups.

Technically we could reuse the ".sd-app" mimetype config elsewhere, but
using a ".default" name for it keeps the door open on divergence in the
future, and matches what we've been testing as part of template
consolidation.

securedrop-workstation-config/debian/postinst

@@ -28,6 +28,7 @@ case "$1" in
       systemctl restart paxctld
       cp /opt/sdw/open-in-dvm.desktop /usr/share/applications/
       cp /opt/sdw/mimeapps.list.sd-app /usr/share/applications/mimeapps.list
+      cp /opt/sdw/mimeapps.list.sd-app /opt/sdw/mimeapps.list.default
     fi
     ;;
 

securedrop-workstation-config/debian/securedrop-workstation-config.install

@@ -1,6 +1,5 @@
 mimeapps.list.sd-viewer opt/sdw/
 mimeapps.list.sd-app opt/sdw/
-mimeapps.list.sd-app opt/sdw/mimeapps.list.default
 mimeapps.list.sd-devices-dvm opt/sdw/
 open-in-dvm.desktop opt/sdw/
 paxctld.conf opt/sdw/