Update duplicate handling

• prevent duplicate list names
• when duplicate lists or entries are created, pretend to succeed and return the id of the existing row instead of throwing an error
• in the case of lists, also update the description if it is different

This obsoletes the message 'readinglists-db-error-duplicate-page' but
it isn't removed as a later patch will be able to reuse the translations.

Bug: T184680
Change-Id: I61c8176cf5913c53c5291872432fafa7415414d3