Add plain JS scroll up button

[ev-sc]

Fixes STW-28

Simple plain JS solution for this request

• Fades in an arrow icon button when a page extends beyond 2x the height of the viewport
• Scrolls to the top of the page on click and fades out

[linear]

STW-29 Infinite repetition of items on scroll, on several pages

The Advice page demonstrates this most clearly, as there are only a couple of items on the page:

https://www.stop-watch.org/what-we-do/advice/

Also happens on the RAW Project

https://www.stop-watch.org/what-we-do/projects/raw/2022-schedule/

And the Research page

https://www.stop-watch.org/what-we-do/research/

and potentially others, but hard to tell on pages with lots of items to scroll through.

72424820-6a0b-434a-98c7-97e104b8b3b4

STW-28 add a 'return to top of page' icon on all pages

by extension, we don't have a 'return to top of page' icon that web users can click on (like this, compatible with the stopwatch look: https://www.dreamstime.com/scroll-up-page-symbol-blue-vector-icon-arrow-to-top-back-classic-flat-image141999140). it'd be great to have one appear at the bottom right hand side of a page as a web user who has scrolled down a webpage beyond the fold.

Originally posted by @HabKadSW in https://github.com/commonknowledge/stopwatch/issues/35#issuecomment-1237077434

[conatus]

Gets it done with the minimum of fuss.