(bug 40810) Add badges support to Item

Need to handle diffs viewing.

PS 2: Added more tests.
PS 3: Badges stubbing.
PS 5: Badges are ItemIds
PS 6: Moved some logic to SimpleSiteLink
      Fixed legacy serialization handling
PS 8: Added some tests

Change-Id: I1c8a88f7013ced4a42abef18b90887ae506e425e