Add 21-11 Fraser River Turbidity image loop var name

The name of the turbidity variable was changed in V2021-11. For now, it still
represents the Fraser River turbidity. Having both the new and old variable name
image loops in the biology image loops data structure enables the page view code
to work across the V2019-05 to V2021-11 transition.

salishsea_site/views/salishseacast.py

@@ -308,6 +308,14 @@
                 svg_name="Fraser_tracer_thalweg_and_surface",
             ),
         ),
+        ImageLoop(
+            model_var="turbidity",
+            metadata=FigureMetadata(
+                title="Fraser River Turbidity Fields Along Thalweg and at Surface",
+                link_text="Surface Fraser River Turbidity",
+                svg_name="turbidity_thalweg_and_surface",
+            ),
+        ),
     ],
 )