[Rails][React] Add PDF document field to Story model

[rudokemper]

Discussed in https://github.com/Terrastories/terrastories/discussions/764

^{Originally posted by rudokemper October 6, 2021}
We have heard from some of our potential end users (i.e. in Colombia) that it would be nice to be able to embed documents like PDFs to stories. For some storied places, there are existing documents that can be added to the story as additional context.

User stories:
As an admin, I can add a PDF file to a story when I first upload it, so that I can provide more rich context to the story.
As an admin, I can add a PDF file to a story from the edit screen, so that I can provide more rich context to the story.

Acceptance criteria:

• Users with admin access see a field called "Documents" in the story editing screen
• Users with admin access see a field called "Documents" in the story upload flow
• Users can upload one or more documents
• Users see a success message once the document is successfully uploaded
• Users see an error message if the document does not upload
• Documents are stored in ActiveStorage