Make scripts more portable

I noticed that the postinstall script failed when trying to set a development environment for the frontend. Other scripts failed as well when trying to run them, relying on yarn bin to find executables in node_modules.

This merge request tries to fix this by

1. not relying on yarn to be globally installed, at the price of some duplication in the package.json
2. changing the bash shebang line in the script to not rely on bash being installed in /bin (see https://stackoverflow.com/a/21613044)