Clean-up - Implement basic CRUD for /voucher-storage/acquisitions-unit-assignments

Overview:
Define and implement the acquisitions-unit-assignments API

use new interfrace: voucher-storage.acquisitions-unit-assignments
See API Listing for endpoint details.
use existing schema created in MODORDSTOR-92
should include standard metadata (createdBy, etc.)
add indices for both recordId and acquisitionsUnitId fields
foreign key on acquisitionsUnitId field and recordId

Additional background can be found on the wiki

Acceptance Criteria:
ModuleDescriptor, RAML are updated
Unit tests are updated

Overview:
Given our change in direction wrt acquisition unit assignments, we no longer need separate APIs/views/etc. for managing acquisition unit assignments for invoices/vouchers

This effectively "undoes" the work done previously in this story

Approach
Typically we wouldn't just remove APIs that have already been released, but given that it's highly unlikely that anyone has started using these APIs yet, that's how we're going to proceed here...
Remove the APIs from:
RAML
ModuleDescriptor
Code

Acceptance Criteria:
The APIs are removed
The View is removed
Unit tests are updated