Introducing MergeItemsInteractor

This factors the common logic of the MergeItems api module
and the MergeItems special page into a separate interactor
class.

This also introduces two test helpers, EntityModificationTestHelper
and ApiModuleTestHelper that are expected to be re-used when we
factor interactor logic out of more API modules.

Bug: 68730
Change-Id: I2987e6875e288632cf9ca3ec8199581b427fc2cd
Merge "Introducing MergeItemsInteractor"