Upgrade to ESLint 4.1 and add no-focused-tests rule (#11977)

* Runs a lint rule on tests only that errors if it sees `fdescribe` or `fit` calls.
* Changes `file:` to `link:` for our custom, internal rules (just to simplify updating these in the future).
* Updates `eslint` from 3.10 -> 4.1 and `babel-eslint` from 7.1 -> 8.0 so that we can run this new rule only against tests.