Added clear cache button to admin settings, updated cache busting for book cover images

cps/static/js/main.js

@@ -405,6 +405,18 @@ $(function() {
             }
         });
     });
+    $("#clear_cache").click(function () {
+        $("#spinner3").show();
+        $.ajax({
+            dataType: "json",
+            url: window.location.pathname + "/../../clear-cache",
+            data: {"cache_type":"thumbnails"},
+            success: function(data) {
+                $("#spinner3").hide();
+                $("#ClearCacheDialog").modal("hide");
+            }
+        });
+    });
 
     // Init all data control handlers to default
     $("input[data-control]").trigger("change");