Use unique keys to identify healthchecks

Summary: Added unique keys for each healthcheck and used them to save already seen failures. Later I will also use them for gathering doctor analytics.

Reviewed By: jknoxville

Differential Revision: D19301583

fbshipit-source-id: 0c0aa977ea73ce555e0d9fb8e8ead844624fb9cd

static/icons.json

@@ -294,5 +294,8 @@
   ],
   "play": [
     16
+  ],
+  "cross-outline": [
+    16
   ]
 }