Convert JsonService and other non-SciJava services into SciJava services

There is a class named JsonService in the ImageJ Server project. This service however is NOT a SciJava service. Since ImageJ Server relies on SciJava, which has got an own service model, the fact that JsonService is not a SciJava service causes certain confusion.

@ctrueden pointed out in https://github.com/imagej/imagej-server/pull/30

> I think part of the confusion is that the Dropwizard stack has its own service model, so the term "service" becomes technically ambiguous in this project. And further adding to this confusion: I don't think the JsonService is a Dropwizard service either! 😆 It would be nice if someone who takes the time to understand both architectures (SciJava and Dropwizard) could clean up this project.

Start by reading JsonService and the other service classes in the ImageJ Server project, then review the discussion in pull request #30. Compare how these classes relate to the SciJava and Dropwizard service models. Done means the project’s non-SciJava services have been consistently converted or otherwise clarified without the current service-model confusion.

Written by the indexing model from the issue text.