Deprecate and remove dashboard code

As stated by jarednorman (in #4860):
> None of this provides any real value. Adding a custom dashboard is
> trivial and I really don't think we need a custom permission set and
> empty controller/view.

backend/app/controllers/spree/admin/dashboards_controller.rb

@@ -3,6 +3,10 @@
 module Spree
   module Admin
     class DashboardsController < BaseController
+      before_action :deprecate
+      def deprecate
+        Spree.deprecator.warn "The #{self.class.name} is deprecated. If you still use dashboards, please copy all controllers and views from solidus_backend to your application."
+      end
     end
   end
 end

backend/app/views/spree/admin/dashboards/home.html.erb

@@ -1 +1,2 @@
 <%# Placeholder view for a home dashboard %>
+<p><%= Spree.deprecator.warn "The Home view is deprecated, If you still use dashboards, please copy all controllers and views from solidus_backend to your application." %></p>

core/lib/spree/permission_sets/dashboard_display.rb

@@ -19,6 +19,8 @@ def category
       end
 
       def activate!
+          Spree.deprecator.warn "The #{self.class.name} module is deprecated. " \
+            "If you still use dashboards, please copy all controllers and views from #{self.class.name} to your application."
           can [:admin, :home], :dashboards
       end
     end

core/spec/models/spree/permission_sets/dashboard_display_spec.rb

@@ -10,6 +10,7 @@
 
   context "when activated" do
     before do
+      Spree.deprecator.warn 'The dashboard_display_spec.rb test is being deprecated.  Please update your source code.'
       described_class.new(ability).activate!
     end
 
@@ -18,6 +19,7 @@
   end
 
   context "when not activated" do
+    Spree.deprecator.warn 'The dashboard_display_spec.rb test is being deprecated.  Please update your source code.'
     it { is_expected.not_to be_able_to(:admin, :dashboards) }
     it { is_expected.not_to be_able_to(:home, :dashboards) }
   end