Switch opencollective images to self-hosted spritesheet

[Munter]

After we added all our opencollective sponsors and backers to the documentation, the load time and the total download size of the documentation increased significantly. It can easily take several seconds to finish loading the opencollective images, despite them being on a CDN served on http2.

This network graph for our current situation looks like this:

There is no way for a user to opt out of downloading these 256 images at about 700kb in total.

• We should increase the speed of download, since these images are literally the first thing people see when visiting the documentation
• We should reduce the amount of bytes downloaded

Description of the Change

The opencollective fetcher script now downloads all sponsor logos and avatar images in order to self-host. A sprite sheet post process is added in order to reduce the amount of requests and further lossy compression is applied.

• Fetch full images and store them. Previously we fetched the first chunk only to figure out dimensions
• Change supporters template to use self-hosted images
• Add a spritesheet definition to make assetgraph-builder sprite the self-hosted images
• Apply lossy compression to the sprite sheet: pngquant 256 colors. Reduces the download size by approximately 8x
• Remove avatar.js, since sprite sheets are background images that don't trigger onload events
• Filtered out personal backers that specifically have asked to be anonymous

Improvements:

• Reduced download size by about 500kb
• Faster time to final render with backer images
• Less CPU time usage (due to removal of JS to image loaded animation)

Here's a before and after comparison: https://www.webpagetest.org/video/compare.php?tests=200607_Z2_58f219880cacb17ae211d74b8e670c1c,200608_DX_cd846fd4961c94ad247dab95eaac76d8-r%3A1-c%3A0&thumbSize=200&ival=500&end=full

Alternate Designs

We could self host the individual images, but speed measurements showed less improvement than a sprite sheet.

Benefits

Faster time to final rendering of the top part of the documentation. Significantly less downloaded bytes

Possible Drawbacks

The backer image loading animation is replaced by a slightly less appealing raw image loading progress. On the other hand it's much faster.

[boneskull]

LGTM if it's ready

[coveralls]

Coverage remained the same at 93.578% when pulling be128cb on munter/sprite-opencollective-avatars into a2f2e08 on master.

[boneskull]

lgtm again. @Munter merge at your convenience; it's not clear to me if you're finished with this one.