Report test coverage achieved by e2e tests to codecov

[felixfbecker]

See https://www.philkrie.me/2018/07/04/extracting-coverage.html

This would allow us to track our progress in automating release testing and identify areas that are untested.

Since e2e coverage often doesn't test all possible cases and are not as rigorous as unit tests, they should be reported with the e2e flag to codecov, so they can be filtered by and show up separately in reports.

This would also allow us to detect unused CSS.

[felixfbecker]

This turns out to be more difficult than expected because Puppeteer does not apply source maps to the coverage. It may be possible to map after the fact but it would need some work.

istanbuljs/puppeteer-to-istanbul#18
puppeteer/puppeteer#3570
istanbuljs/v8-to-istanbul#21

[felixfbecker]

Done