FIX: add protection for scss removal during upgrade

In some cases plugins would remove scss files or change them, but CSS
was still calculated based off stale data in old instance cache

spec/components/stylesheet/manager_spec.rb

@@ -65,6 +65,30 @@ describe Stylesheet::Manager do
     expect(new_link).to include("/stylesheets/desktop_theme_#{theme.id}_")
   end
 
+  describe 'digest' do
+    after do
+      DiscoursePluginRegistry.stylesheets.delete "fake_file"
+    end
+
+    it 'can correctly account for plugins in digest' do
+
+      theme = Theme.create!(
+        name: 'parent',
+        user_id: -1
+      )
+
+      manager = Stylesheet::Manager.new(:desktop_theme, theme.key)
+      digest1 = manager.digest
+
+      DiscoursePluginRegistry.stylesheets.add "fake_file"
+
+      manager = Stylesheet::Manager.new(:desktop_theme, theme.key)
+      digest2 = manager.digest
+
+      expect(digest1).not_to eq(digest2)
+    end
+  end
+
   describe 'color_scheme_digest' do
     it "changes with category background image" do
       theme = Theme.new(