Bug 27072: Don't process staff interface CSS with rtlcss

Bug 27072: Don't process staff interface CSS with rtlcss

This patch changes Koha's gulpfile to add a check for the "OPAC" context
before using rtlcss to automatically build right-to-left versions of
CSS.

To test, apply the patch and test the process of building CSS for both
the OPAC and staff client:

- "yarn css --view opac" : This should build unminified CSS for the
    OPAC, including map files in bootstrap/css/maps

- "yarn build --view opac" : This should build minified CSS for the
  OPAC and generate the RTL versions.

- "yarn css" : This should build unminified CSS for the staff
  interface, including map files in prog/css/maps.

- "yarn build" : This should build minified CSS for the
   staff interface, with no creation of RTL versions.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>