Support --automate-all deployment

[copelco]

Continuing towards #1 -- Adds webapp management to the PythonAnywhere client and implements the full --automate-all deployment flow, including:

• Adds webapp creation/reload methods using pythonanywhere-core's Webapp API
• Modifies settings.py and wsgi.py templates for PythonAnywhere deployment
• Uses python-dotenv to support environment variables (and generates boilerplate)
• Copies the custom WSGI file after webapp creation (since PythonAnywhere overwrites it)
• Adds CI workflow for running integration tests within django-simple-deploy to help catch regressions

Caveat: Currently only tested with django-simple-deploy's test project. Needs more testing with other project structures, like Django Girls.