Link generator: init application type from query params

[consideRatio]

This PR allows the nbgitpuller link generator to pre-populate the application type and urlpath from query parameters, just like hub, repo, and branch can be pre-populated.

# this should prepopulate the application choice
https://jupyterhub.github.io/nbgitpuller/link.html?hub=https://my-jupyter-hub.example.com&app=jupyterlab&repo=https://my-gitlab.example.com/<GROUP or USER>/<REPO>

# this should prepopulate application choices to custom and urlpath to something
https://jupyterhub.github.io/nbgitpuller/link.html?hub=https://my-jupyter-hub.example.com&urlpath=/custom/stuff&repo=https://my-gitlab.example.com/<GROUP or USER>/<REPO>