New "template-parts" folder & Jetpack Infinite scroll

New file template organization + Jetpack Infinite Scroll support by @grappler.
This PR is referring to leopuleo #637, now closed, and discussion #619.

 
This seems somewhat verbose and doesn't actually describe what `_s_infinite_scroll_render` is used for. I think that either `Render the IS Loop` or `Render the Loop` would be perfectly fine here. No comment would also be totally fine, as the function name is pretty clear.