Update the web client to support exporting/downloading stories

[Exolun]

User story

As a user, I want to export/download my stories from the web client, so that I can save them locally or share them with others.

Acceptance criteria

• The story player has a button or menu option to export/download a story
• The story is exported as a .zip file containing all of the relevant assets to experience the story
• The exported/downloaded story has a .html entry point that launches directly to a story player
• The exported story doesn't require a local server running (for client or server)
• You should be able to copy the exported story to another device (such as your phone) and open/view it

Additional notes

This work is likely dependent on some initial refactoring to facilitate deeplinking/routes that open a given story directly in the story player. Also some restructuring of the client side code so that it's easier to point story asset links to a local relative file rather than the web service asset uris, which are currently hacked in.