Subscription application consumes JSON API to show storybooks to users

[waseem]

The API docs can be found at https://github.com/authorly/subscription-backend/blob/master/docs/api.mkd#storybooks.

Behavior

A. When user first time opens the mobile application

1. Make request to /api/storybooks.json with empty or nil last_updated_at.
2. Save the response JSON and last_updated_at value in the JSON locally for later use.
3. Download the thumbnail images for the storybooks whose URL is present in the JSON and display those images on the storybook list screen.
4. When user touches a storybook thumbnail, download the zip file of assets located at asset_url and save it locally. Unzip the zip file and use storybook_json to play the storybook for the user.

B. When user opens the mobile application for subsequent uses

1. Make request to api/storybooks.json with last_updated_at value saved in step 2 in scenario A.
2. Follow step 2 to 4 in scenario A.