Swap splash tiles to use viz screenshots

[trvrb]

preview (also see final form of splash in #922)


This commit swaps the tiles used on the splash page from the previous doctored electron micrographs to be simple screenshots of the pages / pathogens that are linked to. This has two advantages:

1. It's super simple and no longer requires sourcing a copyright-free image and doctoring it in LunaPic
2. It's a more up front, more transparent window into what you get when you click on a tile

I used tree vs map depending on spatial / epi patterns. For pathogens like WNV and Ebola, the map is meaningful and interesting, while for things like H3N2, it's less relevant. I generally tried to stick close to the content that's directly linked and not update coloring, etc..., but I made a couple of exceptions for visual interest.

Checklist

• [x] Replace images on /pathogens
• [x] Remove unused images
• [x] Remove credit from footer
• [x] Downsize images
• [x] Checks pass
• [x] Merge base PR #930

[jameshadfield]

Nice! I was a big fan of the styled cards in the early days but they didn't scale well in multiple senses of the word. This should give us a lot more scope to implement new functionality in future.

Way beyond this PR, if we end up wanting similar tiles in an automated fashion (/groups/x etc) it's possible to automate their creation using a similar approach to how we converted narratives to PDFs

[jameshadfield]

Parent PR (LHS) vs this PR (RHS). Colour matching between tiles and rainbow Nextstrain title is nice.

[victorlin]

Rebasing this onto latest master

[victorlin]

I added a few more things to the PR description to consider if pursuing this direction. Feel free to tweak!

[victorlin]

Rebasing to resolve merge conflicts

[victorlin]

Bundling these changes into #922 (aeffbe28333a75282857ded46c26a30fa6287274)

[victorlin]

Actually there's enough going on in the screenshot swap that I'm going to put those changes back into this PR. This should make it easier to review each batch of related commits.