Allow toggling reading list articles for on/offline.
- Remove any remaining logic pertaining to list-wide offline state.
- Hook up the "action" button in each reading list page item that toggles the page's offline state.
- Add long-press hints and snackbar messages for offline and online state.
TODO: Although this basically implements variant A of the design mocks, we
eventually want to work towards building variant C. This patch is simply
an intermediate step towards that final implementation (and is acceptable
for a maintenance release).