Format all the things (#991)

* Format Javascript and CSS files with `prettier`: `prettier --write 'CTFd/themes/**/*'`
* Format Python with `black`: `black CTFd` & `black tests`
* Travis now uses xenial instead of trusty.

tests/utils/test_sessions.py

@@ -5,9 +5,9 @@ def test_sessions_set_httponly():
     app = create_ctfd()
     with app.app_context():
         with app.test_client() as client:
-            r = client.get('/')
-            cookie = dict(r.headers)['Set-Cookie']
-            assert 'HttpOnly;' in cookie
+            r = client.get("/")
+            cookie = dict(r.headers)["Set-Cookie"]
+            assert "HttpOnly;" in cookie
     destroy_ctfd(app)
 
 
@@ -15,7 +15,7 @@ def test_sessions_set_samesite():
     app = create_ctfd()
     with app.app_context():
         with app.test_client() as client:
-            r = client.get('/')
-            cookie = dict(r.headers)['Set-Cookie']
-            assert 'SameSite=' in cookie
+            r = client.get("/")
+            cookie = dict(r.headers)["Set-Cookie"]
+            assert "SameSite=" in cookie
     destroy_ctfd(app)