Add a referer to know the context of generated story

[anthony-robin]

When the task run and generate a new story or chapter, we should store the referer that initiate the action for better understanding on how things happen.

Possible ways:

• an enum field that would store task or admin
• a dedicated User with task role:
  • Pro: stories belongs to a user_id as a regular model association
  • Cons:
    • We must ensure that we cannot logged in from interface with this user which could make model more complicated
    • More complicated policies with more roles to handle

Might be blocked by #55