Add isLoggedIn check

Summary:
This diff adds a check to ensure the user is logged in.

If they are not, we show an error message until the user logs in.

This will resolve the many posts we get about this such as https://fb.workplace.com/groups/208584557077075/posts/724180735517452/

Reviewed By: nscoding

Differential Revision: D37329064

fbshipit-source-id: efd416f07a0aae73c5e6c47f231419637b3df323

desktop/static/icons.json

@@ -70,7 +70,8 @@
     24
   ],
   "caution": [
-    16
+    16,
+    24
   ],
   "checkmark": [
     16
@@ -478,7 +479,8 @@
     16
   ],
   "caution-circle": [
-    12
+    12,
+    24
   ],
   "megaphone": [
     12,
@@ -634,4 +636,4 @@
   "stethoscope": [
     16
   ]
-}
+}