[WIP] Replace develop.py with manage.py (using django-app-manage)

This Pull Request proposes to remove ``develop.py`` (and it's related functions in ``cms.test_utils``) with a file called ``manage.py`` that can be used like a ``manage.py`` file in a Django project.

While ``develop.py`` works fine, it is a lot of code that has nothing to do with the actual CMS to maintain and it only supports a few commands, new commands need to be added one-by-one. django-app-manage allows to externalize that problem. It's also a project that could be used by 3rd party cms plugins, making testing easier there (this obviously doesn't require this PR to be merged, all this would do is put some weight behind it).

``develop.py`` also set up the magic ``--parallel`` and ``--isolated`` flags for the ``test`` command, but those flags resulted in odd output. For this, I wrote django-better-test which provides those two flags, but keeps the output nice and tidy.

TODO:

* [x] add ``manage.py``
* [ ] remove ``develop.py``
* [ ] remove old code from ``cms.test_utils``