Fixes #813. Replace JSHint with ESLint

[miketaylr]

This add the equivalent linting rules we had before plus a handful of style checks, mostly around whitespace.

Running grunt will still run the eslint task, or you can run grunt eslint for only that.

One cool thing is you can tell it to "fix" the errors for you (for certain simple ones, like whitespace or indentation):

eslint webcompat/static/js/lib/user-activity.js --fix, for example.

r? @magsout

[magsout]

cooool. Good job @miketaylr

One cool thing is you can tell it to "fix" the errors for you (for certain simple ones, like whitespace or indentation):

I don't like a linter modifies files, I think it's better to display an error or warning.

[miketaylr]

I don't like a linter modifies files, I think it's better to display an error or warning.

That's fair. By default it only displays errors. But if you have 10,000,000,000 whitespace errors, you have the option of manually using that --fix option. 😄

[miketaylr]

OK cool, tests are passing (we had some weird timeout thing). Let's pull it in.

[miketaylr]

Ooops haha.

[magsout]

That's fair. By default it only displays errors. But if you have 10,000,000,000 whitespace errors, you have the option of manually using that --fix option. 😄

Sounds good. Good job 👍