Models with string typed primary keys violates integer field in the hooks

[javrasya]

Description:

As @xuziheng1002 has reported in issue #127;
Since the object id fields in the hook model classes are positive integer field, it is impossible to have models with string primary key field.

Acceptance Criteria:

• It should be Charfield instead of PositiveIntegerField as it is in Transition and TransitionApproval.
• Prepare a migration script that would safely migrate the existing object id's to its string values.