remove the venv activation from npm scripts

[Pomax]

We added npm scripts for activating the python virtualenv and then running mezzanine, but unfortunately this only works on operating systems that understand the source command. This means that the npm scripts cannot run on windows (nothing exists that does what source does for bash, on windows), and so running everything with a convenient npm script cannot work (not even in theory).

We should remove the automatic venv activation and runserver command from the npm scripts, and find a way to make the commands necessary to run the mezzanine server the same irrespective of the OS used.

[gvn]

How about we add x-platform support instead of removing something currently useful for the majority of developers in the interim?

[cadecairos]

The only possible cross platform solution I know of right now is virtualenvwrapper (virtualenvwrapper-win for windows)

if we made it a requirement for development, we can put something like this in the setup:

Create a virtual environment with virtualenvwrapper: 
  - OSX/Linix `mkvirtualenv network -p python3`
  - Windows `mkvirtualenv network -p C:\Python36\python.exe`

Then, in an npm script we can run workon network and it will activate the virtual environment regardless of the platform.

[Pomax]

pipenv fixed this.