Parse 'reviewed' event object 'state' property

[ghost]

The Timeline Events API can return a reviewed event object when listing events for a pull request. One of the properties of this object is state, which tells you the state of a submitted review.

The values state can have are:

• commented
• changes_requested
• approved

This PR adds this field to the Timeline struct defined in issues_timeline.go to make it possible to tell the state of a review when listing pull request events.

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[ghost]

@googlebot I signed it!

[ghost]

Merging this PR would fix issue #1885.

[gmlewis]

Thank you, @campesel !
LGTM.

Awaiting second LGTM before merging.

[ghost]

@gmlewis Thanks for the approval!

[ghost]

@wesleimp Looks good to you?

[gmlewis]

Friendly ping @wesleimp

[Parker77]

LGTM.