Introduce an agency environment variable

[thekaveman]

So that we know in logs and events if we ever combine those from multiple agency sources.

[thekaveman]

Plan is to handle this with #253, because we'll want to set a value for all sentry events to indicate which agency the event came from.

Idea is to add an agency_name property that reads from environment, in the usual place in settings.Configuration class, which can be customized by implementations in the settings.py file.

agency_name values will correspond with agency.long_name from Benefits, which is also used in analytics/errors.

Attach a custom tag to Sentry events in Python.