Add infinite scroll to Twitter results

[paulgibbs]

[pepegar]

I've been working on this today. It's not hard, in fact is really easy because the functionality of appending the next set of tweets is already implemented by the Load more button. The bad part is that we need to override the same method of the wp.media.view.MEXP view for every service we want to implement infinite scroll, and that's not possible with our architecture.

My proposal is moving the infinite scroll support to mexp.js, and giving the service the option to implement infinite scroll or still using the Load more button for loading more content.