Change DBHOST and DBUSER forms

[kraigb]

Also use pathlib.Path instead of os.path in settings.py.

Purpose

• Removes complexities and customer failures in the App Service tutorial caused by needing a username@servername form for the credentials given to Postgres. With this change, the required username form is created automatically from DBHOST and DBUSER, rather than having to be specified explicitly.
• Also changes DBHOST to be only the server name, not the full server URL, as the latter is easily constructed by appending the standard domain info for Postgres on Azure.

Does this introduce a breaking change?

[X] Yes (explained in the updated README.md)
[ ] No

[kraigb]

NOTE: Do not merge until updates to the associated tutorial are verified as published.