https://artsyproduct.atlassian.net/browse/PLATFORM-1136

Problem:

One part of the product health matrix story [spike][spike] is code
coverage for each service. However, there is no pattern at Artsy for
measuring code coverage for Javascript applications.

While there are existing tools out there for tracking and reporting code
coverage, such as [istanbul.js][istanbul-js], [coveralls][coveralls],
and [codecov][codecov], the complexities:

1. A multi-runner test suite (jest and mocha), and
2. A Circle CI -> Docker -> Docker test runtime

have resulted in a tough problem to solution around.

Solution:

After spending a lot of time trying to get Coveralls instrumentation to
work, we pivoted to using Codecov. At a high level, this commit does the
following:

1. Updates our calls to `jest` and `mocha` to write code coverage
information to a `.nyc_output` directory (default for nyc/istanbul)

2. Manipulates the `jest` coverage information to be compatible with
`mocha` coverage information via a JS script. See [motivating GH
issue][gh-issue].

3. Merges the `jest` and `mocha` coverage information together using the
`nyc` CLI

4. Introduces a bash executable provided by codecov to instrument the
merged information to codecov

Co-authored-by: Kieran Gillen <[email protected]>

[spike]:https://artsyproduct.atlassian.net/browse/PLATFORM-1098
[istanbul-js]:https://github.com/istanbuljs/nyc
[coveralls]:https://coveralls.io/
[codecov]:https://codecov.io/gh/artsy/positron/
[gh-issue]:jestjs/jest#2418

* Add BRANCH_NAME to .env.test so that it's whitelisted for Docker
* Pass through CIRCLE_BRANCH to `hokusai test`
* Extend expression to `codecov` to take a branch name