Improvements after running through Django Girls tutorial

[copelco]

Add environment validation error messages and Django Girls tutorial-specific references:

• Implements _validate_platform() to check for required API_USER and API_TOKEN environment variables before deployment
• Adds API connectivity test to fail fast with helpful error messages if credentials are invalid
• Expands README with step-by-step Django Girls tutorial deployment instructions
• Updates sequence diagram and documentation to clarify bash console behavior (browser must open console URL to start the process per PythonAnywhere API requirements)

TODO:

• Run webapp.reload() AFTER copying wsgi file
• Configure webapp's static path for /static/ to point to /home/$USER/my-first-blog/static, somehow dynamically find that

refs #11