Fix image aspect ratio in homepage sliders on mobile, debounce a scroll event

[benlk]

Changes

This pull request makes the following changes:

• Changes the image aspect ratio on the homepage sliders on mobile so they aren't stretched vertically
• Debounces the thing that checked whether to show/hide the button to smooth-scroll the browser to the top of the page, because that was firing every scroll tick and never showing the button as a result. Now it should show up sometimes, with much less thrashing.

Why

For #3

Testing/Questions

Features that this PR affects:

• On mobile, the homepage top section should change from the first image to the second image:
• on browsers more than ~1300px wide, in the lower right, a button should appear.

Questions that need to be answered before merging:

• [x] Is this PR targeting the correct branch in this repository?
• [ ] do we want to fiddle with the scroll behavior of the button?

Steps to test this PR:

1. check out the branch.

[joshdarby]

On mobile, the homepage top section should change from the first image to the second image:

Looks so much better now.

do we want to fiddle with the scroll behavior of the button?

I hope not 😬

Debounces the thing that checked whether to show/hide the button to smooth-scroll the browser to the top of the page, because that was firing every scroll tick and never showing the button as a result. Now it should show up sometimes, with much less thrashing.

Works well!