Investigate client-side code coverage

[LeeDr]

Describe the feature: UI tests are slow and can be flaky so we should not add more of them unnecessarily. Having code coverage metrics could help us plan tests to get the right coverage.

Describe a specific use case for the feature: Here's some links about Istanbul tool that might be useful;
https://istanbul.js.org/

https://www.engineyard.com/blog/measuring-clientside-javascript-test-coverage-with-istanbul

https://medium.com/@the1mills/front-end-javascript-test-coverage-with-istanbul-selenium-4b2be44e3e98

[LeeDr]

cc @silne30 @spalger @tylersmalley Maybe we could work together on this on a fix-it-friday?

[LeeDr]

@legrego says he's used Istanbul before

[LeeDr]

This is in place now. Closing.