Interactive demo assets base path fix (#975)

## Changes
* Make asset past constant with respect to root of PennyLane Website
* This fixes assets not loading for [interactive
demo](https://pennylane.ai/qml/demos/circuits_as_fourier_series/)


## Note 
- this might not work correctly with current QML preview, but will work
on react site.
- this will work with #967

_static/demos/circuits_as_fourier_series/sketch.js

@@ -1,11 +1,4 @@
-var url_prefix = '../../_static/demos/circuits_as_fourier_series/src/';
-
- fetch(url_prefix + 'fourier0-1.png', {method: 'HEAD'}).then(res => {
-     if (!res.ok) {
-         url_prefix = '../_static/demos/circuits_as_fourier_series/src/';
-     }
- });
-
+const url_prefix = '/_static/demos/circuits_as_fourier_series/src/';
 
 ///// Sketch 0