use the new mofo-style ESLint rc file

[Pomax]

with MozillaFoundation/mofo-style#24 landed, we should tap into this new and improved linting solution.

[Pomax]

A big problem here is that our new ESLINT rules are primed for "use ES6", which means that right now there are about 5000 errors pertaining it "please use arrow functions instead".

As discussed with @simonwex, one way forward here may be if we can somehow selectively apply ESLINT only to files that are updated after we turn on ESLINT linting, but this is an unresolved issue at the moment.

[simonwex]

I'd like to talk through this during our next Engineering call. I've added it to that agenda.

[Pomax]

clearing the milestone here, because this is a very tricky problem, and is not happening without us figuring out how to do this for our entire app ecosystem.

[Pomax]

mostly done, finally. three rules remain, they're tracked in a newer issue.