Fixes old migrations w/ trackers.

This adds the old tracker model to the migrations that use it. The code
for the migrations was not changed, as the const name is set to Tracker
via

  Object.const_set("Tracker", AddTrackerPosition::Tracker)

Each migration removes the Tracker constant afterwards with

  Object.send(:remove_const, :Tracker)
Adds migration that renames trackers to types.