Added new parent context

refs https://github.com/TryGhost/members.js/issues/5

- Adds new parent context with default values for all common context data we want to pass through

This adds base for using contexts (https://reactjs.org/docs/context.html) in our application as that allows easier overall base data and method sharing between different components instead of passing them down

ghost/portal/src/components/ParentContext.js

@@ -0,0 +1,10 @@
+// Ref: https://reactjs.org/docs/context.html
+const React = require('react');
+
+export const ParentContext = React.createContext({
+    site: {},
+    member: {},
+    action: '',
+    brandColor: '',
+    onAction: () => {}
+});