Polyfill function.prototype.bind

So... Although this PR goes against our core beliefs and especially against mine, there is a valid, data driven, reason to this madness.

We recently added `Function.prototype.bind` to our cut-the-mustard test, which inadvertently kicked out ~4% of users from receiving our core JavaScript application. This took that demographic from 6 to 10%, and broke our line in the sand to serve JS to ~95% of users.

This PR introduces a `polyfill.js` which includes one for `bind()`, and a strongly worded comment.

This is a temporary change going forward, as we are discussing the ability to separate analytics & advert code from our core JS file. More to follow in the coming weeks.

/cc @ironsidevsquincy @gklopper @tackley