Grid field detail form can't write relations

[sminnee]

When a grid field is editing a many-many-through relation, the "join object" will be available on each record via DataObject::getJoin()..

Although it's currently a bit awkward and requires boilerplate such as this in your DataObject, you can then open these records up for editing in your edit form:

    public function getNewImpactID() {
        if ($this->joinRecord instanceof RiskReduction) {
            return $this->joinRecord->NewImpactID;
        }
    }
    public function setNewImpactID($value) {
        if ($this->joinRecord instanceof RiskReduction) {
            return $this->joinRecord->NewImpactID = $value;
        }
    }

The problem is that write() by default doesn't write components. Triggering this requires one of the flags passed to write being set here: https://github.com/silverstripe/silverstripe-framework/blob/4/src/Forms/GridField/GridFieldDetailForm_ItemRequest.php#L615

I'd like to set the flag. We could either set it in all cases (which would let people modify any has_one relationship on a grid field edit form), or just for objects that have a join record (which would limit the bugfix to the place I tripped over it, possibly prevent current code that relies on this bug from being broken, but risking that we need to change this code a second time.

[sminnee]

My instinct is to set writeComponent = true in all cases, as it's safe and will make the framework more flexible, especially if #9163 is also implemented.

[sminnee]

I'm going to need to re-introduce this as a config option rather than a change to the default, assuming #9519 is merged into 4.5, which seems to be the consensus.

Should I target 4.5 or 4.6+?