Add permission to view duplicate images

[mdashlw]

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• I understand my contributions may be rejected for any reason
• I understand my contributions are for the benefit of Derpibooru and/or the Philomena software
• I understand my contributions are licensed under the GNU AGPLv3

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Continuation of #412
Depends on #427

Adds a new permission "View duplicate images" that allows regular users to :show and :index duplicate images. All other interaction is blocked, except for :vote which depends on :show.

Image API will return full information about the images for authorized users. Search API will only return duplicates if ?del=1 is specified (handled by ImageLoader).
A side-effect of API now doing can?(user, :show, image) is that mods/admins can now use their api key and view (non-duplicate) deleted images.

This depends on a new Role for which there's no migration mechanism so it needs to be created manually:

alias Philomena.Roles.Role
%Role{name: "duplicate_access", resource_type: "Image"} |> Role.changeset(%{}) |> Repo.insert()