This is a regression introduced somewhere along the way in the new storage
API system for osTicket.

  * Include a `bk` column to store the storage backend
  * Include a `signature` column which represents a repeatable hash of the
    file contents
  * Rename `hash` to `key` since it isn't a real hash

This will help distinguish between the knowledgebase part which encompasses
FAQ's and canned responses, and the FAQ part which contains the articles
detailing frequently-asked-questions.