refactor: split optimized images into WebP and optimize audit

[patrickhulce]

closes #2044 and does some drive by cleanup of byte efficiency audits

Splits the audit to track two concerns separately:

1. Are you doing the most sensible optimizations for your existing image format?
   JPEG doesn't have an insane amount of metadata or is too high quality, can be expanded to do the equivalent of PNG crush too.
2. Are you using the most efficient image format for your content?
   Currently just a WebP check but could be fancier for your PNG could actually be a JPEG, etc.

Also reduces the aggressiveness of both of these by increasing the quality to 85 and increasing the ignore threshold to 4KB for JPEG and 8KB for WebP.

Questions:

• Should the WebP check be moved down into diagnostics? No
• Should we rename "Optimize images" since it's just tracking the lowest hanging fruit optimizations, if so, to what?

screenshot: (added by paul)

[ebidel]

This makes sense to me. I think we should keep the audits in "Opportunities" (not move them to Diagnostics) as they are potential improvements to be made.

One thing, can you group the 3 image audits together, probably in this order:

1. Properly size images
2. Optimize images
3. Convert images to WebP

"Convert images to WebP" could also be "Serve images as WebP" to suggest that this would be in addition to serving other formats. But that would also address Kayce's concerns in the bug.

[ebidel]

Needs a rebase.

[patrickhulce]

also turns out we've been hiding offscreen-images from the perf section this whole time, so I added that back

[patrickhulce]

@ebidel PTAL rebased and updated order :)

[paulirish]

I'm really happy with this split. Good call.

I like the title of both of these audits now. seems good.