Refactor the effect on the sponsors and fix a position caching bug

[skirtles-code]

I was investigating #195. This doesn't necessarily fix all the problems reported there but it may help.

The Patreon Sponsors on the homepage are shown in grayscale until the page has scrolled to a certain point. I made the following observations:

1. The cached value for sponsorTop is not always correct. Elements can move when content loads, it doesn't require a page resize for things to move. This could be why Sponsor gray/color change is jumpy #195 observed the transition occurring in a strange place.
2. The listeners were never removed. This didn't actually cause any errors but they do leak when you navigate away to a different page.
3. The code directly manipulates DOM elements rather than using Vue to perform the updates.
4. I don't know why there's a check for window. The hook is called during SSR so that shouldn't be necessary.

I've refactored it to use Vue in a more idiomatic way. This also removed a big chunk of the code. The caching of sponsorTop is gone and the listener is removed when leaving the page.

I don't believe there was any significant performance benefit from caching the offsetTop value. Style recalcs and page reflows shouldn't be a factor here.

Aside from fixing the caching bug, I have not changed the point at which the transition occurs or the speed of the transition.