Basic Doctor UI

Summary:
- Basic Doctor UI showing issues with installation
- Run healthchecks in background on startup and show warning message if something is wrong

Reviewed By: jknoxville

Differential Revision: D18502599

fbshipit-source-id: 194939a080ba7412ed3293d95c533bfad7031d3b

src/utils/icons.js

@@ -45,6 +45,9 @@ const ICONS = {
   bug: [12],
   camcorder: [12],
   camera: [12],
+  caution: [16],
+  cross: [16],
+  checkmark: [16],
   desktop: [12],
   directions: [12],
   internet: [12],