small errors during setup

[nerdoc]

I try to be precise without using the issue template.

Using a current django-sockpuppet installation, fresh done using the setup "tutorial". There are a few problems in there, some smaller, but I think, to make it easier for novices (like me), I'd change a few things - I'll list them here in one issue.
I'm using Python 3.9, Node 14.15.3, npm 6.14.9, webpack 5.18.0, webpack-cli 4.4.0

1. npm install missing. After initial_sockpuppet (js bootstrapping), a npm install run should be recommended in the docs, or done automatically by the mgmt cmd.
2. python manage.py initial_sockpuppet installs add-project-script@1.0.2 globlly, and uninstalls it afterwords again without asking, assuming it was not installed before by the user, but fails if it was installed before, e.g. with yarn. npm -g installs don't work on my machine as npm seems to have problems putting the binary into place. yarn global add works fine, but breaks the initial_sockpuppet script. Be more error tolerant here.
3. At the beginning of the script, there should be a check if tools like npm or a callable add-project-script are available.
4. initial_sockpuppet creates invalid "watch" script (at leasr for webpack 5): it adds "watch": "webpack --watch --info-verbosity verbose" to package.json. But --info-verbosity doesn't seem to be a correct option of webpack (5?).