Make switching projects in RStudio work

[yuvipanda]

This PR contains a number of fundamental imrovements!

• We supervise the rsession process, restarting it if it stops with a non zero exit code. This is just good practice anyway, and it also allows 'projects' functionality in rstudio to work

• We just proxy everything via '/rstudio/' rather than redirect people to the /proxy// URL.

  This provides a number of benefits:

  • Users never see a 'Connection Refused' error, since we check to make sure the process is up before forwarding them through
  • Consistent URL rather than exposing internal detail of what port is being used!
  • Much better experience when switching projects - you get a small message that says 'switching projects', then a spinner then it exists!
  • Make sure that port is shared across requests, rather than a new one being allocated per request! This worked before by accident :)
  • Use trailing slash when accessing rstudio (so rstudio/ than rstudio), to make relative URLs used by rstudio work properly. This too worked before by accident
  • Don't make a HTTP request to check before each GET. This is not necessary, since we know the process is up (since we watch for SIGCHLD and delete the proc key if it is dead)

[yuvipanda]

This requires https://github.com/jupyterhub/nbserverproxy/pull/9 to work!