Add targetPath to link generator.

[ryanlovett]

This fixes #326. jupyter-shiny-proxy can look for shiny apps in alternative locations like ~/ShinyApps/. We need our instructors to be able to set this in targetPath via the form.

I tested this by building the docs locally and a JupyterHub link worked for me.

[ryanlovett]

@GeorgianaElena @consideRatio @yuvipanda Would it be okay if I merged this myself? I'll wait another week for people to raise any objections.

[consideRatio]

Looks right from a quick mobile phone look through!

👍 on self-merging this kind of change when review capacity is low. Pinging and waiting is a bit before is a suitable pattern i think.

[ryanlovett]

Thanks @consideRatio !