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

static/compilePlugins.js

@@ -39,7 +39,7 @@ module.exports = async (
   }
   watchChanges(plugins, reloadCallback, pluginCache, options);
   const dynamicPlugins = [];
-  for (let plugin of Object.values(plugins)) {
+  for (const plugin of Object.values(plugins)) {
     const dynamicOptions = Object.assign(options, {force: false});
     const compiledPlugin = await compilePlugin(
       plugin,