Do not report errors to Sentry in Django shells

Closes #759 (closed)

There is no easy way to tell Sentry to ignore exceptions whose stack trace contains django.core.management.commands.shell or something that comes from its REPL. I had made multiple tests earlier with before_send hooks and analyzing the traceback, but it's a mess. The only way I found to not enable Sentry on Django shells is to look for shell or shell_plus in the command line, and just not initialize Sentry at all when it's there.