chore: use pngquant to optimize PNG images

[dsanders11]

Uses imagemin-pngquant to optimize PNG images at build time.

Significantly reduces the size of images used on the home page, shrinking transferred size from 4.4 MB to 1.2 MB.

It's technically lossy optimization, but I don't think anyone can tell the difference between this and this unless you've got Superman eyes.

Before:

After:

[ckerr]

Unless there's a compelling reason to use a lossy compressor like pngquant, can we use one of the several lossless ones instead?

zopflipng, optipng, advpng, and pngcrush are all lossless options that have npm wrappers.

This script is overkill for electron/website but does show invocation recipes for trying to maximize lossless compression from these four tools

[dsanders11]

The compelling reason is the significant reduction in size for imperceptible visual change.

I briefly tied a lossless optimizer on one of the larger files as a test and it was something like a 15% reduction, compared to 75% with pngquant.

[erickzhao]

Great idea. LGTM once @BlackHole1's comment is addressed. :)

Addressed

[dsanders11]

Had an offline discussion with @ckerr regarding his comment, and moving ahead with taking the easy win here and we can follow up with other improvements in the future, such as committing losslessly optimized versions of the files to begin with. 👍