Remove public stories.

[cproctor]

Currently, users who are not signed in can create "public" stories which can be edited by anyone. The idea was that in casual workshops we could get people started right away without having to create accounts. However, this feature is just waiting to be abused, and we will have a better "quick-start" alternative for workshops via student mode: We could just create a new teacher account just for the workshop, and give participants a QR code with the join code.

Therefore, public stories will stop being a thing.

• Update UI to remove the option to create a story when not logged in.
• Create a migration to remove public stories from the database.
  • Any stories which are forked from a public story should have their parent relation nulled.
  • Delete all public stories.
• Create a migration to remove the public attribute from stories.
• Update the story model manager (which implements permissions) to remove checks for public stories.