Implement chains and next/prev links

[aelzenaar]

On /v/ pages, the next/prev links should point to (for example) the next/previous day with articles. (This will require us to pass the list of items in each db entry around.)

Inside the database, there will be a `chains' table which points to the first article in each chain; then each item in the chain will be linked (1) back to the chains table entry, and (2) to both the next and previous items as a doubly-linked list.