spike: experiment with codecov's "merge reports"

[vanderhoop]

As a developer,
I want to have our front-end and backend code coverage merged into a single report
So that I and pull-request submitters have a holistic view of their coverage.

By Way Of Explanation: at the moment, our Coveralls integration is only tracking coverage for Elixir (backend code), as Coveralls doesn't support multiple reports (or merging multiple reports) for a given build. While our frontend coverage is solid, folks who submit pull requests aren't automatically notified if their coverage goes down in the front-end, because we have yet to set this up. And this results in a delay between submission and feedback.

Acceptance Criteria:

• open branch with codecov integration, surfacing coverage/deficiencies across both the backend AND frontend

Helpful Links:

• CodeCov: Merging Coverage Reports
• CodeCov: Example Elixir Project
• CodeCov: Example Node Project