ensure app can run on paas

instead of using wsgi, we now use "application" - this tells gunicorn
to look inside the python module application (application.py) for a
wsgi app - and by default that is also called application so rename
the variable.

Also, when running tasks, we're not using gunicorn so need to set the
flask variable in the manifest so that `flask command ...` finds the
app properly.

Remove server_commands as it's not used any more.

application.py

@@ -5,6 +5,6 @@ from flask import Flask
 
 from app import create_app
 
-app = Flask('app')
+application = Flask('app')
 
-create_app(app)
+create_app(application)