Use a linked_instruments in the WS2 controller

All the other entity types have a linked_... function in
MB::S::ControllerBase::WS::2 for loading additional entities related
to the main entity, so use the same model for instruments.
MBS-7744: Return alias type for all entities in WS2

Some entities (areas, instruments, labels, places, and series) had
specific code in their web service 2 controller for loading aliases,
instead of relying on the generic code automatically provided by
MB::S::ControllerBase::WS::2. This extra code did not provide any
additional features and, unlike the generic code, did not load alias
types, preventing them from appearing in the WS output. It is
therefore simply removed.
Add test for MBS-7744
Call collections collections instead of lists

Collections had a linked_lists function instead of linked_collections,
possibly for some historical reasons. Rename to avoid future
confusion.
Merge remote-tracking branch 'chirlu/mbs-7744' into beta

* chirlu/mbs-7744:
  Call collections collections instead of lists
  Add test for MBS-7744
  MBS-7744: Return alias type for all entities in WS2
  Use a linked_instruments in the WS2 controller