Twenty Fifteen: Replace frontend jQuery usage with vanilla JS

[flixos90]

Using jQuery for a few simple JS interactions in WordPress themes is a common performance problem, which is in most cases relatively straightforward to address: Vanilla JavaScript has for the better part of a decade included enough APIs that work across all popular browsers so that the remaining benefits of jQuery are around a few utility functions, that could be re-implemented in a few lines of code. Using jQuery for such simple use-cases results in around 80% more JS code being loaded in the frontend.

This ticket focuses on removing jQuery as a frontend dependency for Twenty Fifteen - specifically the frontend, as for example in the Customizer jQuery is loaded anyway (and performance is a bit less of a concern there).

Also see #54171, which does the same for Twenty Twelve.

• Replace jQuery usage in Twenty Fifteen frontend scripts with vanilla JS.
• Don't require jQuery to be loaded in the frontend as a result of the above.

Trac ticket: https://core.trac.wordpress.org/ticket/54172