EST-257: removing Lockable interface itself
EST-260: Party#bankMandates and #leases...

... should return the BankMandates and Leases, respectively, rather than
the AgreementRoles.

Doing this by introducing AgreementRoleHolder, and contributed services
(BankMandateContributions, LeaseContributions), contributing to this interface
EST-260: BankMandates#newBankMandate made programmatic

... because we already have Lease#newBankMandate as the way to setup bank mandates

Also:
- BankMandates#allBankMandates (prototype) now moved under FinancialAccounts menu