Add EntityRevision functionality

Having a way to retrieve an entity along with revision meta info
from a storage service will allow us to move away from using
WikiPage and Revision for retrieving entities.

This would provide a unified storage layer at least for retrieval,
avoids global state and improves testability. It also paves the
way for the introduction of sub-entities.

Change-Id: Ia9aff0add78c036546dce804ad7147d8d589082d
Merge "Add EntityRevision functionality"