Expose date_gmt for view context of Posts and Comments

[danielbachhuber]

It can be useful for determining the absolute time for the resource.

date_gmt was set for edit context only in
29c768b and
a12c23a

See #1675

[danielbachhuber]

@WP-API/amigos #reviewmerge

@rachelbaker Do you recall why you moved comment_date_gmt to edit context in 29c768b ?

[rachelbaker]

@danielbachhuber the original philosophy I was trying to adhere to was is this property used for front-end public display? I have never seen a theme that displayed the post_date and separately also post_date_gmt.
That being said, I notice that we do expose modified_gmt for Posts....so we should be consistent one way or the other. I am 👍 with including the _gmt dates in the view context.