Add code coverage PR comments and badge to repo

[jcqli]

Look into adding code coverage reports to the repo. Can it be done across both repos?

• [ ] Move coverage to top level of repo so that it includes web/ and functions/
• [ ] Set up PR codecov.io comments (https://docs.codecov.com/docs/pull-request-comments)

[gino-m]

We have this in Android, just need to set up on this repo: https://github.com/google/ground-android/blob/master/codecov.yml