Add MIME type for RELAX NG files to enable syntax highlighting

RELAX NG [1] is a schema language for XML.

Map the .rng extension to the XML MIME type.

[1] http://www.relaxng.org/

Change-Id: Iafe7f39bd38b09a3ff6242c8becb9aa9db455de7

gerrit-server/src/main/resources/com/google/gerrit/server/mime/mime-types.properties

@@ -36,6 +36,7 @@ properties = text/x-ini
 py = text/x-python
 r = text/r-src
 rb = text/x-ruby
+rng = application/xml
 rst = text/x-rst
 scala = text/x-scala
 soy = text/x-soy