Pool pubsub connections for notifications (#1626)

* Add a `listen()` method to `CTFd.utils.events.EventManager` and `CTFd.utils.events.RedisEventManager`. 
  * This method should implement subscription for a CTFd worker to whatever underlying notification system there is. This should be implemented with gevent or a background thread.
  * The `subscribe()` method (which used to also implement the functionality of the new `listen()` function) now only handles passing notifications from CTFd to the browser. This should also be implemented with gevent or a background thread. 
* Pool PubSub connections to Redis behind gevent. This improves the notification system by not having a pubsub connection per browser but instead per CTFd worker. This should reduce the difficulty in deploying the Notification system.
* Closes #1622 
* Make gevent default in serve.py and add a `--disable-gevent` switch in serve.py
* Revert to recommending `serve.py` first in README. `flask run` works but we don't get a lot of control.
* Add `tenacity` library for retrying logic
* Add `pytest-sugar` for slightly prettier pytest output

serve.py

@@ -1,4 +1,3 @@
-from CTFd import create_app
 import argparse
 
 parser = argparse.ArgumentParser()
@@ -6,7 +5,20 @@ parser.add_argument("--port", help="Port for debug server to listen on", default
 parser.add_argument(
     "--profile", help="Enable flask_profiler profiling", action="store_true"
 )
+parser.add_argument(
+    "--disable-gevent",
+    help="Disable importing gevent and monkey patching",
+    action="store_false",
+)
 args = parser.parse_args()
+if args.disable_gevent:
+    print(" * Importing gevent and monkey patching. Use --disable-gevent to disable.")
+    from gevent import monkey
+
+    monkey.patch_all()
+
+# Import not at top of file to allow gevent to monkey patch uninterrupted
+from CTFd import create_app
 
 app = create_app()