Added configurable HIDDEN_STACKTRACE_MODULES

Using dirname on SocketServer was wiping out all of the system libraries from the stacktrace, since it is a single-file module unlike Django.

But it is also nice to be able omit things like gunicorn, threading, newrelic, etc as required -- otherwise the DOM structure generated for the stack trace blows out to impractically large proportions with a large number of queries.

May be nice to also add regex filter capabilities for modules which cannot be imported (e.g. gunicorn entry points)