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 it if the description is different
Also remove some old schema updates to avoid autodetection conflicts.
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.