Bring ImagePreviewActivity into ImageViewController.swift and improve implementation.

Previously, a lame ad hoc "uses the last NSURL in the activity items" convention identified which of several possible URLs might locate an image. Instead, we'll box it up into an instance of a class that only the ImagePreviewActivity will look for.

Bonus: some bits of formatting in ImageViewController.
Add a "Copy Image URL" activity.

Appears in the image preview, as well as when long-tapping a linked image in a Message or a Post.