Include `Linked issues` section in `clean-sidebar` feature #2773

PRs and Issues have a new sidebar section that should also be "cleaned"

<img width="239" alt="" src="https://user-images.githubusercontent.com/1402241/74312347-7e7c2400-4dac-11ea-800c-544245ef2524.png">

• Hide the "Successfully merging a pull request may close this issue"
• Hide "None yet"
• If you can't edit the list, and it's empty, completely hide the section (no info is shown and no action can be taken, so why show anything at all?)