DC2B-34 Supporting multiple user types, adding search function.

1) Add Organization domain class.
2) Add Controller for Organization.
3) Modify User domain class:
     - change the relationship with UserRole from manyToMany to oneToMany.
     - Add organization filed.
     - Do necessary modifications correspond to this change.
4) Add Search function for Listing Users.
5) Add a DB migration changelog file for DB schema changes.