Make STN settings consistent with other apps

[maeve-fpf]

There are a few quirks in each app's settings that we'd like to make a little more unified. Things we can do on STN:

• Remove the HSTS settings, other apps leave this to the TLS termination level
• base.py should default DEBUG to False so that we can reason about it in the same way as other apps
• Stub out CSP variables in the same manner as SDO so that we can use them later
• Simply the process of adding whitenoise to middleware (follow SDO and PFT, instead of complex calculating-an-index logic)
• Instead of DJANGO_LOG_CONSOLE and DJANGO_LOG_PATH, just log to console if a log path is not set. Don't require DJANGO_LOG_JSON, default it appropriately.
• Treat env vars that should be "booleans" the same everywhere.