Custom microbit loader (#334)

* Custom microbit loader

* updated animation

* use standardized name

theme/site/elements/loader.variables

@@ -1,3 +1,8 @@
 /*******************************
     User Variable Overrides
 *******************************/
+
+@loaderSpeed: 2s;
+@loaderAngle: 5deg;
+
+@large   : 200px;