Fix donation banner bugs

- The HEIM_DONATION_URL variable was not properly propagated into the
  build.
- The donation store would race with the localStorage one for the
  latter's load event.

client/gulpfile.babel.js

@@ -36,6 +36,7 @@ const heimOptions = {
   HEIM_PREFIX: process.env.HEIM_PREFIX || '',
   EMBED_ORIGIN: process.env.EMBED_ORIGIN,
   NODE_ENV: process.env.NODE_ENV,
+  HEIM_DONATION_URL: process.env.HEIM_DONATION_URL,
 }
 
 // via https://github.com/tblobaum/git-rev

client/lib/stores/donations.js

@@ -13,6 +13,7 @@ const store = module.exports.store = Reflux.createStore({
     }
     this.listenTo(storage.load, this.onStorageLoad)
     this.listenTo(actions.sendMessage, this.onMessageSend)
+    if (storage.store.state !== null) this.onStorageLoad()
   },
 
   getInitialState() {