Define global logger instance to be used for all logging

Summary:
The advantage of this is that anyone can now log without needing to carry around a Logger object.
Primary reason is for making metrics less intrusive in the codebase.

Reviewed By: passy

Differential Revision: D13671368

fbshipit-source-id: 162ab1351a124683aef13983fc3bcec01385cd1a

src/fb-stubs/BugReporter.js

@@ -5,7 +5,7 @@
  * @format
  */
 
-import LogManager from './Logger';
+import type LogManager from './Logger';
 
 import type {Store} from '../reducers/index.js';