Forces use of gedit for text files via mailcap

This may be a larger problem with the Debian 9 transition: xdg-open
when run via a terminal (rather than invoked via e.g. Nautilus
integration, or otherwise in a GUI desktop environment) does not
honor the mimetype handlers declared in the desktop files. For
text/plain, edited the mailcap file in order to address.

dom0/sd-svs-files.sls

@@ -55,6 +55,13 @@
     - mode: 644
     - makedirs: True
 
+# xdg-open falls back to run-mailcap on debian-9, so CLI invocations
+# will not use gedit as intended. Let's force use of gedit via mailcap.
+sd-svs-force-gedit-for-xdg-open:
+  file.prepend:
+    - name: /etc/mailcap
+    - text: "text/plain; gedit %s; test=test -x /usr/bin/gedit"
+
 sudo update-mime-database /usr/local/share/mime:
   cmd.run