Modify `manage.py clean` to be pure Python

Currently, the clean function in manage.py uses system commands to clean the codebase. Since it supports both Windows and Linux, there are two sets of commands being used. This should be written in pure Python instead to only have one piece of code that works on all OS'.

Code being referenced:
https://github.com/StackFocus/PostMaster/blob/master/manage.py#L50
