Refactor app container file system and enable WEBSITES_ENABLE_APP_SERVICE_STORAGE

[thekaveman]

This is a follow up to #1763

Now we can customize the location that Django creates and looks for its Sqlite database file 👍 But we can't write to a persistent storage location, because our app service container overwrites the Azure-provided persistent file system each restart. So it can't be used to maintain our database yet.

Further, due to the way our Docker image build structures the app's files, we can't take advantage of this local storage -- both would be competing to co-exist under /home. We need this directory to remain stable for the Azure filesystem, and we need the app to execute from a different directory.

Acceptance Criteria

• the Docker image build sets a non-/home based directory as the root of the app container
• WEBSITES_ENABLE_APP_SERVICE_STORAGE is true
• App config django-db-dir is /home/site/wwwroot

Additional context

Azure docs: Use persistent shared storage

Potential alternative to #1773

[thekaveman]

Trying #1773 first because that would be simpler / fewer changes to the app structure.

[thekaveman]

Closing this as it looks like #1773 works!

We can re-open later if we want to try this approach.