Remove jquery dependency from site.

Rewrite code using pure js (es6). Focus should be on small, fast code.

[x] split global.js into separate smaller files
[x] setup a new gulp task for merging & transpiling the files.
[x] ensure the customizer preview doesn't get caught up in things.
[x] rewrite new merged files in vanilla js.
[x] borrow skip-link-focus from twentynineteen
[x] Can we replace scroll_to_hash with a small vanilla equivalent or should we just ditch it?
[x] Do we need some javascript to make the navigation work properly on touch & old browsers?

BinaryMoon / jarvis