Update various Xplat Prettier configs to specify arrowParens: avoid

Summary:
Since Prettier 2.0, the default for the `arrowParens` option has changed from `avoid` to `always`.

In preparation for upgrading Prettier in Xplat to 2.1, add `arrowParens: 'avoid'` explicitly to configurations that were missing it so that we do not have a change of behaviour when upgrading.

Changelog: [Internal]

Reviewed By: bolinfest

Differential Revision: D23666397

fbshipit-source-id: 665f20d1419d0d664a6ef5cddc46f441b20eeb51

Author: gkz

.prettierrc

@@ -3,5 +3,6 @@
   "singleQuote": true,
   "trailingComma": "all",
   "bracketSpacing": false,
-  "jsxBracketSameLine": true
+  "jsxBracketSameLine": true,
+  "arrowParens": "avoid"
 }

template/_prettierrc.js

@@ -3,4 +3,5 @@ module.exports = {
   jsxBracketSameLine: true,
   singleQuote: true,
   trailingComma: 'all',
+  arrowParens: 'avoid',
 };