Set public_until instead of clearing public_on when unpublishing elements

Previously, setting `public = false` on an element cleared its `public_on`
timestamp, turning it into a draft. Now it sets `public_until` to the current
time, preserving the publication history while marking the element as expired.
Setting `public = true` now also always refreshes `public_on` and clears any
`public_until` value.

(cherry picked from commit 73a290c)

Author: tvdeyen

app/models/alchemy/element.rb

@@ -234,9 +234,10 @@ def store_page(page)
     def public=(value)
       @public_on_explicitely_set = true
       if ActiveModel::Type::Boolean.new.cast(value)
-        self.public_on ||= Time.current
+        self.public_on = Time.current
+        self.public_until = nil
       else
-        self.public_on = nil
+        self.public_until = Time.current
       end
     end
 

spec/models/alchemy/page/publisher_spec.rb

@@ -28,7 +28,7 @@
     shared_context "with elements" do
       let(:page) do
         create(:alchemy_page, autogenerate_elements: true).tap do |page|
-          page.draft_version.elements.first.update!(public: false)
+          page.draft_version.elements.first.update!(public_until: 1.minute.ago)
         end
       end
     end