Fixed a changeset migration issue

In 1.3 ChangesetUsers were not getting cleared in the following scenario
* Changeset was created
* Add content to the changeset
* Delete the changeset.
The dependencies were not properly specified in the changeset model.
The model has been hence been fixed.
This commit clears the dirty data in ChangesetUsers table during migration
before creating the new foreign key constraints.
Merge pull request #3353 from parthaa/changeset-migration

Fixed a changeset migration issue