Added business rules validatation for unassign loan officer

Hello Keith,

In this commit updates are as below

* Updated api doc for assign and unassignLoanOfficer feature
* moved /loans/{loanId}/assign & /loans/{loanId}/unassign to /loans/{loanId}?command=assignLoanOfficer / removeLoanOfficer => I will modify UI also accordingly
* Added util to get LocalDate by MFI's timezone
* Added timezone table mifosplatform-tenants only for reference and not linked to tenant table
* Modified tenants.timezone_id column's type to varchar (as tz is referred with names)
* For unassignLoanOfficer added two business rules ( #1 can't unassign on future date #2 can unassign in past but not beyond assigned date )

Thanks
Nayan Ambali