Parameterize autograder URL (#1286)
Parameterize autograder URL (#1286)

Enabling the `AUTOGRADER_URL` value to be set by deployments of the
application means that we don't have to rely on a single autograder
deployment and other OK users such as Imperial College can deploy their
own instances.

Note that it's a bit of a misnomer to have the `AUTOGRADER_URL` in the
constants file but dereference it from an environment variable. Likely a
more consistent place would be the Flask app settings. However, the
autograder is implemented outside of the Flask app context so it might
be a non-trivial change to pull the configuration from the Flask
settings and it would unnecessarily couple the two. As such, reading the
environment variable in the constants file seems like a pragmatic
compromise.
Cal-CS-61A-Staff/ok,Cal-CS-61A-Staff/ok,Cal-CS-61A-Staff/ok,Cal-CS-61A-Staff/ok,Cal-CS-61A-Staff/ok