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

src/NotificationsHub.js

@@ -355,7 +355,7 @@ const shadow = (props, hover) => {
   if (props.inactive) {
     return `inset 0 0 0 1px ${colors.light10}`;
   }
-  let shadow = ['1px 1px 5px rgba(0,0,0,0.1)'];
+  const shadow = ['1px 1px 5px rgba(0,0,0,0.1)'];
   if (props.isSelected) {
     shadow.push(`inset 0 0 0 2px ${colors.macOSTitleBarIconSelected}`);
   }