Install and deploy pathagar book server

I made some changes to reference implementation in plugins.d/, please take a look and discuss them:


1) Custom settings are managed in `prod_settings.py` using Python inheritance mechanism:

```python
# prod_settings.py
from settings import *

DEBUG = False  # Overrides from upstream value
...
```


2) Pathagar is installed in a Python **virtual environment** but inheriting packages from the global site-packages directory. This allows to install specific version for some packages using pip (f.e. Django) but use the system ones (f.e. python-psycopg2) for others.


3) For deployment, instead of using Apache mod_wsgi, it uses gunicorn WSGI server. Pathagar is running standalone on a specific port. It is possible to reverse proxy from Apache to have schoolserver.local/pathagar or to use mod_wsgi.