Add previous/next reading order fields to ReadingList

[bpepple]

Summary

• Add self-referential previous/next FKs to ReadingList so curators can chain lists into a larger reading order (e.g. "Batgirl: Mother" -> "Batgirl: The Book of Shiva"), with save() keeping both sides of the link in sync automatically
• Expose the links in the admin, the create/edit form (with a new ReadingListAutocomplete), the reading list detail page, and the API's ReadingListReadSerializer ({id, name})
• Add a "Clear selection" button next to the Previous/Next autocomplete fields, since the third-party widget has no built-in way to unset a single selection

Closes #565