Activate virtualenv in production Python code.

The manage.py change effectively switches the Zulip production server
to use the virtualenv, since all of our supervisord commands for the
various Python services go through manage.py.

Additionally, this migrates the production scripts and Nagios plugins
to use the virtualenv as well.

scripts/lib/setup_path_on_import.py

@@ -0,0 +1,15 @@
+"""
+Use libraries from a virtualenv (by modifying sys.path) in production.
+Also add Zulip's root directory to sys.path
+"""
+
+import os
+from os.path import dirname, abspath
+import sys
+
+BASE_DIR = dirname(dirname(dirname(abspath(__file__))))
+activate_this = os.path.join(BASE_DIR, "zulip-venv", "bin", "activate_this.py")
+if os.path.exists(activate_this):
+    # this file will exist in production
+    exec(open(activate_this).read(), {}, dict(__file__=activate_this)) # type: ignore # https://github.com/python/mypy/issues/1577
+sys.path.append(BASE_DIR)