Invenio-circulation

- fixes `initial_loan`, a copy of loan before transition, to be a local
variable instead of a Transition object property.

- adds config CIRCULATION_LOAN_LOCATIONS_VALIDATION to allow validation
of loan locations

- fixes the way the default module configuration is defined to allow
overriding it from the Invenio app

- renames signals parameter from `prev_loan` to `initial_loan`
- adds an extra parameter `initial_loan` to the default duration functions
in checkout policies

- adds additional checks when transitioning loans from and to Item at Desk
- removes example app and updates Invenio modules dependencies

- introduces `pid_type` for `item_pid` to uniquely identify an Item record
- changes methods that were expecting `item_pid`
- removed unused view circulation/items to retrieve the item from the loan