Maintain revision history of package metadata updates

[pranavkm]

From https://nuget.codeplex.com/workitem/3348

Whenever a package's metadata is changed or the package is unlisted, a revision history should be kept. And this revision history should be queryable from NuGet.org.

The process of unlisting or altering package metadata should require whoever is doing it to supply an explanation that becomes part of the revision history. Ideally, when a package is unlisted, this explanation will become the Description field saying something like "This package has been deprecated. Please use package X.Y instead".

Users should be able to see this revision history and explanations at least from the Data Services, and maybe eventually in Package Explorer or Package Manager.

[TimLovellSmith]

Great idea to have better support for deprecation of packages/package versions.
Do you have an intended purpose/use for the entire history of past list/unlist events, or only for the explanation 'why is this package I'm interested in unlisted'?

[analogrelay]

As we look at more editable metadata in #565, we also agree that having a history of changes is good. So we'll do that when we work on editable metadata.