Reenable codecov reports on PRs

### Description
Currently we run mocha for some tests in the app/ folder, and custom code in development/merge-coverage converges the two different test runner's coverage output into a single map. This is then uploaded to codecov AND is used for our circle ci to validate coverage numbers.

However, there seems to be some discrepancy in how this is done as the results of our internal tool is sometimes weirdly off relative to the changes made. In addition, the internal tool report and the codecov report differ on the same PR in many cases. For this reason we must wait until we migrate from mocha to jest in app/ folder. Once that's done we can turn on the codecov checks once again. First we will turn on the status check then we can test the 'patch 'check to see if the line comments match the actual line numbers in the PR!

Start by reading development/merge-coverage and the existing CircleCI coverage flow for tests in app/. Confirm that the Mocha-to-Jest migration is complete before re-enabling Codecov. Done means the Codecov status check is active and its patch comments match the actual changed line numbers in a pull request.

Written by the indexing model from the issue text.