Multisite: Propagate date change to descendants

[arthurperton]

This fixes #3440.

I made these choices:

• The date set on an 'origin' entry will automatically also be set on any of its descendant entries*
• If any of the descendants have revisions enabled, a working copy will be made when needed
• The published state for the origin entry won't be propagated to its descendants

* Note that this won't take into account if the (custom) date field has been desynced. The date will be copied regardless.

[arthurperton]

Where should the tests for these changes go?

[jasonvarga]

I think the propagating should be happening in Entry@save, just like the other propagating stuff.

That way, it can happen if you were to save programmatically instead of only in the control panel.

The tests can go into EntryTest. There are other propagation tests in there.

[jasonvarga]

This works, thanks for making the changes.

However after thinking about it some more, I don't think it should work this way.

Nowhere else do we forcibly update the descendants.

I think the real fix should be something more inline with how origin fallback values work for all other fields.
i.e. If you set a date on the localized entry, it should be saved into the filename like it does now. But if you want to inherit the date, then the filename should just not have the date.

[arthurperton]

Makes total sense.