bootstrap.py contains logic that is only applicable post-generation

[jgonggrijp]

The bootstrap.py contains the activate_frontend function, which is only relevant to the person initially creating the project using the cookiecutter. Once generated, another developer who clones the project need not (and should not) run the steps in this function. So this logic belongs in the post_gen_project.py instead. This is probably also true of the override_package_json function. There might be more of such code that should be moved from the bootstrap.py to the post_gen_project.py.