Add generic data migrator

In order to be able to migrate the json configurations of dashboards,
this change adds a way to easily get access to the fields. The
implementaion makes it easy to extend the provided helpers in the future
to change other fields or tables as well.

Example usage:

    Migrator.run Migrator::Widget do |widget|
      widget['foo'] = widget['bar']['baz']
    end
Add migration to move global stacked setting

With the introduction of multi-axes support, the stacked option was
moved.