Install gevent-websocket and use it by default until we have a better solution (#850)

* Install `gevent-websocket` and use it by default until we have a better solution
* May help with #849

CTFd/config.py

@@ -209,8 +209,8 @@ class Config(object):
         Example: /ctfd
 
     SOCKETIO_ASYNC_MODE:
-        Specifies what async mode SocketIO should use. The simplest but least performant option is 'threading'.
-        Switching to a different async mode is not recommended without the appropriate load balancing mechanisms
+        Specifies what async mode SocketIO should use.
+        Specifying your own async mode is not recommended without the appropriate load balancing mechanisms
         in place and proper understanding of how websockets are supported by Flask.
         https://flask-socketio.readthedocs.io/en/latest/#deployment
     '''
@@ -219,7 +219,7 @@ class Config(object):
     SQLALCHEMY_TRACK_MODIFICATIONS = (not os.getenv("SQLALCHEMY_TRACK_MODIFICATIONS"))  # Defaults True
     UPDATE_CHECK = (not os.getenv("UPDATE_CHECK"))  # Defaults True
     APPLICATION_ROOT = os.getenv('APPLICATION_ROOT') or '/'
-    SOCKETIO_ASYNC_MODE = os.getenv('SOCKETIO_ASYNC_MODE') or 'threading'
+    SOCKETIO_ASYNC_MODE = os.getenv('SOCKETIO_ASYNC_MODE')
 
     '''
     === OAUTH ===

docker-entrypoint.sh

@@ -2,6 +2,7 @@
 set -eo pipefail
 
 WORKERS=${WORKERS:-1}
+WORKER_CLASS=${WORKER_CLASS:-geventwebsocket.gunicorn.workers.GeventWebSocketWorker}
 ACCESS_LOG=${ACCESS_LOG:--}
 ERROR_LOG=${ERROR_LOG:--}
 
@@ -40,6 +41,6 @@ echo "Starting CTFd"
 exec gunicorn 'CTFd:create_app()' \
     --bind '0.0.0.0:8000' \
     --workers $WORKERS \
-    --worker-class 'gevent' \
+    --worker-class "$WORKER_CLASS" \
     --access-logfile "$ACCESS_LOG" \
     --error-logfile "$ERROR_LOG"

requirements.txt

@@ -20,6 +20,7 @@ netaddr==0.7.19
 redis==2.10.6
 datafreeze==0.1.0
 gevent==1.3.7
+gevent-websocket==0.10.1
 python-dotenv==0.9.1
 flask-restplus==0.12.1
 pathlib2==2.3.2