Use ConfigManager during setup.py, not hack to add line to .py config file

[parente]

Here's the dashboards PR jupyter/dashboards#156 that removed the hack. Unfortunately, this switch breaks compatibility with declarative widgets unless they too are added to the JSON config file. Otherwise, notebook says:

[W 02:43:26.010 NotebookApp] Collisions detected in jupyter_notebook_config.py and jupyter_notebook_config.json config files. jupyter_notebook_config.json has higher priority: {
      "NotebookApp": {
        "server_extensions": "<traitlets.config.loader.LazyConfigValue object at 0x7f5dfc0c7f60> ignored, using ['urth.dashboard.nbexts']"
      }
    }

For the time being, I've reverted the fix in the 0.2.x branch of dashboards to retain compatibility but have left the change in dashboards master. We're going to need to change in lock step for this to work.

Source: https://github.com/Anaconda-Server/nbsetuptools

[lbustelo]

Fixing #165 will include this issue.