Added admin screen for the pizza orders

Closes #89

Short description

Adds an admin screen for the pizza orders, such that the organising party can easily manage which pizzas are paid.

Steps to use this feature

1. Open the pizza event.
2. Press the 'Pizza' button.
3. Press the settings cog in the app bar.
4. Manage pizza orders in the same way that you can manage event registrations.

__tests__/reducers/__snapshots__/pizza.spec.js.snap

@@ -2,6 +2,11 @@
 
 exports[`pizza reducer initially should return the initial state 1`] = `
 Object {
+  "admin": Object {
+    "loading": false,
+    "orders": Array [],
+    "status": "initial",
+  },
   "event": null,
   "hasLoaded": false,
   "loading": false,