Implement code coverage metrics for e2e tests

[patcon]

Re-ticketed from @colinmegill's 💬 Gitter chat message

Our e2e testing tool apparently has support packages for figuring out code paths that are covered by e2e tests.

• GitHub Issue thread
• Cypress Code Coverage Guide

To Do

• [ ] investigate feasability for our React codebase
• [ ] ...

EDIT: Would prob help with a big PR like this: https://github.com/pol-is/polis/pull/421