Improve Lazy Image Loading by using IntersectionObserver

Based on the following discussion and improvement plan:

Upgrade Lazy Image Loading by using IntersectionObserver wherever it is available.

Extension: When we are already working on it, take a look how hard it would be to have a manual update mode, so through a method we can deactivate constant DOM Observation and only do it only manual. Could be quite a performance saver on DOM intensive pages like MR/Issue page.