Utilities: Add useExperimentalFlag hook

This commit DRYs out the code, extracting the logic for setting the
value of the experimentalFlag into a new hook found in the utilities.

It also makes the typing stricter - the hook returns a boolean. The
pattern we were using previously to set the value of experimentalFlag
variables could be boolean, string, or undefined.

src/Components/sharedComponents/ImageBuilderHeader.tsx

@@ -25,7 +25,7 @@ import { resolveRelPath } from '../../Utilities/path';
 import './ImageBuilderHeader.scss';
 
 type ImageBuilderHeaderPropTypes = {
-  experimentalFlag?: string | true | undefined;
+  experimentalFlag?: boolean;
 };
 
 export const ImageBuilderHeader = ({