Our long term goal with asset publishing is to make it a completely
separate step from build. It's an operation which can optional be done
after builds complete. Even being done multiple times if needed to say
work around errors.

Additionally publish should be a deliberate, explicit operation. The
simple act of branching should not publish by default, it should be
explicitly opted into.

This moves us in that direction by doing the following:

- Centralizes our publish story to a single file: PublishData.json in
master
- Makes publish a step that can easily be run on any completed build
- Makes NuGet / VSIX publishing per branch and configurable. Allows for
uploads to many feeds if necessary
- Begins using dotnet/versions to track our packages.