Prefer const wherever possible

Summary:
Non-final identifiers make code harder to understand.
This is particularly true for JavaScript where even the *type*
can change as a value gets reassigned later.

This enforces to use `const` whereever possible, but doesn't
"outlaw" `let`. Mixed destructuring is also still allowed.

Used `eslint --fix` to change all existing cases.

Reviewed By: jknoxville

Differential Revision: D16131329

fbshipit-source-id: 2eceaca7c603b71b36e005be5d135e1849f2518d

headless/index.js

@@ -146,7 +146,7 @@ async function exitActions(
   store: Store,
 ): Promise<void> {
   const {metrics, exit} = userArguments;
-  for (let exitAction of exitClosures) {
+  for (const exitAction of exitClosures) {
     try {
       const action = await exitAction(userArguments, store);
       if (action.exit) {