Add automated clean-up for unpublished stories

Once server-side autosave has been implemented (SJRK-289), there should be a task created to automatically clear out any unpublished stories that are older than a certain amount of time.

Consideration should be given to how old a story is before it is determined to be "too old", as we don't want to accidentally delete something that an author has been chipping away at. Since the SJRK-289 work will be adding timestamp values for first created date and last modified date, we'll have to decide which to use to calculate this duration.

Design questions:

• How old should a story be before it is determined to be "stale"?
• What timestamp should we use to determine staleness? Last modified? Created?
• What should happen to localStorage copies of stale stories that have been cleared out?