- all sandbox actions a made from the same endpoint, so i renamed it as
  WebappSandboxPanel. It is an endpoint that accept GET and POST methods
- GET is rendered using TemplateView (nothing magic)
- POST is has an ad-hoc implementation that handles sandbox
  actions (start, commit, rollback, retry, abort)
- all POST actions are followed with an HTTP redirect (to make a GET)
  so that actions are repeated if the user refresh his browser window
- I changed the initial status of newly created webapps to IDLE because
  power users who upload their docker image do not need to start a
  sandbox

It is not finished, there are many FIXMEs/TODOs but it is working.