Adding a shared `is_embargoable` example spec
Adding `is_embargoable` to generic file spec
Moving embargoable element to permissions partial

This is a simple move, but the next steps are to ensure that the embargo
release date's behavior and its user input align. Presently, it looks as
though I can embargo something independently of the Permissions I
assign. In reality, the embargo is only applicable on "Open Access"
items.