Track the remix count and nth remix values for a map

* remix_count - integer representing the number of times a given map
  has been remixed. Equivalent to the result of the following query:
  `SELECT count(*) AS "count" FROM "maps" WHERE ("remixed_from_id" = 1) LIMIT 1`
* nth_remix - integer representing the order in which a remix occured.
  The first remix of map A will have an nth_remix value of 1, the
  second remix of map A will have an nth_remix value of 2, and so on