Refactor _makeBadgeData -> coalesceBadge (#2859)

`base.js` is pretty long and `_makeBadgeData` is one of the most complex
functions in it.

It's a pure function so it's easy to test in isolation. This moves the function into its own module and reorganizes the tests in a way that makes it evaluate what it's doing, and easier to test what is and isn't covered.

Ref https://github.com/badges/shields/pull/2796#discussion_r249251008