Merge django reusable-apps

The primary aim of the work done for this project is to improve the maintainability of OpenWISP. OpenWISP comprises of various modules but overtime we learned that the division of modules was not done in the most efficient manner possible causing maintenance overhead. To avoid this overhead, I have to refactor and rewrite alot of code to merge modules and clean-up the older practices.

This clean-up opportunity can also be used to improve the extensibility of the django modules, the documentation and the existing code with the best practices to improve code readability and make usage easier.