(bug 62643) Split up ChangeOpFactory

This splits ChangeOpFactory into several factories,
each providing ChangeOps for one aspect of entities,
such as terms, claims, sitelinks, etc.

This also introduces ChangeOpFactoryProvider, which
provides ChangeOpFactories based on the type of the
target entity. This allows for validation to be
specific to a given type of entity.

Change-Id: Ifc00c97ecc41ebc9219f5e0e5558c7a777a88e89
Merge "(bug 62643) Split up ChangeOpFactory"