Some older browsers do not ship with isIntersecting, while they already
have IntersectionObserver support. We make use of `intersectionRatio`
now to fix the Lazy Loader for those browsers.

Every browser which supports IntersectionObserver will now use it over
observing scroll and resize events. Older browsers without support fall
back on the previous behavior.

Additionally the MutationObserver can be enabled and disabled manually
via the helper method `startContentObserver` and `stopContentObserver`.
This might prove useful on pages where we manipulate the DOM
extensively.

This stops the checkElementsInView method from running when there aren't any lazy load images on the page. By calling this method we are causing an unnecessary reflow which on large pages is a massive performance issue