Add an integration for newsletter subscription

[pkarw]

Connected to #26. Newsletter claim should be stored in localstorage (you can add sth like "claims Collection" in /store/index.js:21)

Vue.prototype.$db = {
  ordersCollection: localForage.createInstance({
    name: 'shop',
    storeName: 'orders'
  }),

  categoriesCollection: localForage.createInstance({
    name: 'shop',
    storeName: 'categories'
  }),

  cartsCollection: localForage.createInstance({
    name: 'shop',
    storeName: 'carts'
  })
}

global.db = Vue.prototype.$db // localForage instance

Then add the logic to new store "user" added under store/modules.
Finally, adding new claim should execute SW trigger to send this claim out to the server as orders are synchronized (service-worker-ext.js for details), sth like this should be executed:

import * as sw from 'lib/sw'
sw.postMessage({ config: config, command: types.USER_NEWSLETTER_SIGNUP }) // add user to newsletter

in service-worker-ext.js

  if (event.data.command === 'user/NEWSLETTER_SIGNUP') {
...
}

And then - of course you should add new resource type under vue-storefront-api