Implement "Archiving Repositories" functionality

[ryangribble]

As per this recent blog post there is new API functionality to archive repositories.

This touches on several areas of the API

• Add Archived field to PullRequest response model
• Add Archived field to the request model UpdateRepository used to edit a repository (and note that un-archiving a repo can not be done via the API!)
• Add Archived flag to the request model SearchRepositoriesRequest used to search repositories (should render as archived:true in the generated search string)
• WebHooks now have an Archived and Unarchived event, so we should check whether your activity classes need to have this added to EventInfoState enum (I think possibly not though, as this enum is maybe only used for Issue events and not Repository events)

[hnrkndrssn]

Looks like this was implemented in #1845

[ryangribble]

Thanks @hnrkndrssn

Closed by #1845