This moves the four npm download services into a single class, in line with #3174, and other service implementations we've written in the last couple months.
1. Change the suffixes from **/m** to **/month** for consistency.
2. Add tests of one of the intervals besides total.
3. Rewrite mocked service tests as unit tests.
4. Use (and work with) the `intervalMap` pattern that I think @calebcartwright started us using.
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