Remove need to manually run init.sh script for better user experience

[cchwala]

With the split into a content repo and and environment repo in #31 came the problem that we always have to manually run a init.sh in the binder environment before the actual notebook can be run. The script also takes approx. 1 minute to run. Currently what you get as "landing page" in the binder env looks like that:

We could potentially remove the need to run the init.sh script when the binder env is run from the main branch, which is what happens when a user clicks on the launch binder buttom from the README in the sandbox repo.

We could do that by not using the environment defined in the separate env repo which does not allow us to run something like the init.sh script when starting the binder pod. When using the env defined in this repo here we could (if I recall correctly) run either postBuild or start script.

I have not figured out the details yet. I just want to document the idea here since we currently aim at improving the user experience.