Containerized versions of the service will now get their LVAlert
credentials from the environment.  Also add a script for
processing environment variables and starting LVAlert overseer.

Some additional code cleanup, as well.

Clean up the logic and add some examples for X509 cert auth with
impersonation proxies.  Fully implement the corresponding unit tests.

Make the event view 403 pages more specific (for event views only),
but more generalized (standard message in the template, not taken
from the context).  Also fix up some unit tests which expected the
old behavior.

New API backend which gets a full X509 certificate, verifies it,
and extracts the subject.  To be used in the cloud deployment
with Traefik.