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
