Check map & layer sharing when updating dashboard sharing

[wayfaringrob]

When you update the sharing for a map, it flags any layers that are more restricted than the map's setting.

However, dashboards don't do this. Depending on how much you've got in there, it might not be immediately apparent what needs to be changed, and of course you have to go chase those things down manually. It'd be great if DBs did the same things maps did, thanks.