Content update notification

[kelson42]

If a local content has a corresponding online update, then a notification should be troggered with an easy way to download the new version.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be now be reviewed manually. Thank you for your contributions.

[kelson42]

Depends on kiwix/libkiwix#638

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be now be reviewed manually. Thank you for your contributions.