enhanced check to prevent multiple betterlink.js

Don't inject an additional Betterlink script if there is a script
element in the DOM that looks to pull the betterlink.js source. Not a
complete solution: doesn't account for script loaders that *will* insert
Betterlink, but haven't yet.
prevent injection on betterlink.io homepage

At the moment, there's a problem if we load multiple versions of
*different* Betterlink instances on the same page. Because the
betterlink.io homepage uses betterlink-bespoke.js to customize the
experience, injecting the vanilla betterlink.js before the customized
version runs may be unwanted. For now, this prevents that experience
specifically.
fix whitespace