FIX: when destroying uploads clear card and profile background

There is an fk to user_profile that can make destroying uploads fail
if they happen to be set as user profile.

This ensures we clear this information when destroying uploads.

There are more relationships, but this makes some more progress.

spec/models/upload_spec.rb

@@ -413,4 +413,25 @@ describe Upload do
       "https://#{SiteSetting.s3_upload_bucket}.s3.amazonaws.com/original/1X/#{upload.sha1}.#{upload.extension}?acl"
     )
   end
+
+  context '.destroy' do
+
+    it "can correctly clear information when destroying an upload" do
+      upload = Fabricate(:upload)
+      user = Fabricate(:user)
+
+      user.user_profile.update!(
+        card_background_upload_id: upload.id,
+        profile_background_upload_id: upload.id
+      )
+
+      upload.destroy
+
+      user.user_profile.reload
+
+      expect(user.user_profile.card_background_upload_id).to eq(nil)
+      expect(user.user_profile.profile_background_upload_id).to eq(nil)
+    end
+
+  end
 end