Copied fbjs keymirror into utils (#4807)

* Removed react-addons-pure-render-mixin from NOTICE

* Copied FBJS keymirror into non_npm_dependencies

webapp/package.json

@@ -16,7 +16,6 @@
     "intl": "1.2.5",
     "jasny-bootstrap": "3.1.3",
     "jquery": "3.1.1",
-    "keymirror": "0.1.1",
     "marked": "mattermost/marked#69736482dbad685c398a5eec33a59b5ab06057ac",
     "match-at": "0.1.0",
     "object-assign": "4.1.0",