Upgrading flow and fixing lint errors

Summary:
Upgrading to flow 0.91, fixing a bunch of `$FloxFixMe`s that were introduced by upgrading to 0.86.
Also fixing some linting issues.

Reviewed By: priteshrnandgaonkar

Differential Revision: D13900794

fbshipit-source-id: 5d0a1b62371f3b5d34b909bae0876583acb6f977

static/transforms/index.js

@@ -20,7 +20,7 @@ function transform({filename, options, src}) {
     filename,
     plugins: [
       'jsx',
-      'flow',
+      ['flow', {all: true}],
       'classProperties',
       'objectRestSpread',
       'optionalChaining',