Since we've upgraded production to Node 12 (#5436) we can finally adopt static fields!
This starts the process by updating core and one of the service families.
I find having these in a consistent order makes the services much faster to read.
This is the order I’ve generally been using:
1. Category
2. Route
3. Examples
4. Rendering
5. Other helpers (`fetch()`, `transform()`)
6. `handle()`
This switches to the new mozilla add-ons API.
It's not a completely like-for-like replacement. I was able to replicate the version, users and rating badges but we no longer have total download stats. We've got weekly downloads instead, so I've redirected `/d` to the new `/dw`
closes#3079
