[FIX] website: refactore parallax snippet (perf / browser compatibility)

* Do not read multiple DOM data on each scroll. Simply read this when
necessary (edition / initialization).
* Throttle scroll event and debounce resize events (-> performance in
edition and navigation improved).
* Do not save useless data in the DOM.
* Have the parallax state consistent if the page undergo other changes
* Simplify DOM structure and LESS
    - Allow to better see how to place snippets inside the area
    - Before this commit, default parallax had different height on
    chrome and firefox