Ports the Nexus service to the new service model. Some related/relevant conversation in #2347 (and closes#2347). Also adds support for authentication which resolves#1699.
This adds a badge for collaborator count. When evaluating a library, it can be useful to know that there's not a single-contributor bottleneck for publishing. Having more than one collaborator is a sign of library maturity.
It adds another badge for dependency version of published dependencies, which solves a similar problem as the node-version badge. I will find this useful for making sure dependencies are up to date in a library.
This starts the rewrite of the dynamic badges. I've pulled into BaseService an initial version of the query param validation from #2325.
I've extended from BaseJsonService to avoid duplicating the deserialization logic, though it means there is a bit of duplicated code among the three dynamic services. The way to unravel this would be to move the logic from `_requestJson` and friends from the base classes into functions so DynamicJson can inherit from BaseDynamic. Would that be worth it?
This introduces a regression of #1446 for this badge.
Close#2345
