FIX symbiote#309 Support ManyManyThroughList in EditableColumns

The ManyManyThrough fields must be specified without comma, hence its
name must be unique in the joined set of fields. For example, given the
following DataObjects:
```
class Team extends DataObject
{
    private static $many_many = [
        "Supporters" => [
            'through' => TeamSupporter::class,
            'from' => 'Team',
            'to' => 'Supporter',
        ]
    ];
}
class Supporter extends DataObject
{
    private static $belongs_many_many = [
        'Supports' => Team::class,
    ];
}
class TeamSupporter extends DataObject
{
    private static $db = [
        'Ranking' => 'Int',
    ];
    private static $has_one = [
        'Team' => Team::class,
        'Supporter' => Supporter::class,
    ];
}
```
you should refer to the ranking field with `Ranking`, not with
`TeamSupporter.Ranking`.

src/GridFieldEditableColumns.php

@@ -24,6 +24,7 @@
 use SilverStripe\ORM\DataObjectInterface;
 use SilverStripe\ORM\FieldType\DBField;
 use SilverStripe\ORM\ManyManyList;
+use SilverStripe\ORM\ManyManyThroughList;
 
 /**
  * Allows inline editing of grid field records without having to load a separate
@@ -81,7 +82,14 @@ public function getColumnContent($grid, $record, $col)
             }
             $field = clone $field;
         } else {
-            $value  = $grid->getDataFieldValue($record, $col);
+            $value = $grid->getDataFieldValue($record, $col);
+            if (is_null($value)) {
+                // Try to find the value into the join record
+                $join = $record->getJoin();
+                if ($join) {
+                    $value = $grid->getDataFieldValue($join, $col);
+                }
+            }
             $rel = (strpos($col, '.') === false); // field references a relation value
             $field = ($rel) ? clone $fields->fieldByName($col) : new ReadonlyField($col);
 
@@ -150,7 +158,7 @@ public function handleSave(GridField $grid, DataObjectInterface $record)
                 }
             }
 
-            if ($list instanceof ManyManyList) {
+            if ($list instanceof ManyManyList || $list instanceof ManyManyThroughList) {
                 $extra = array_intersect_key($form->getData(), (array) $list->getExtraFields());
             }
 
@@ -235,7 +243,7 @@ public function getFields(GridField $grid, DataObjectInterface $record)
                 }
             }
 
-            if (!$field && $list instanceof ManyManyList) {
+            if (!$field && ($list instanceof ManyManyList || $list instanceof ManyManyThroughList)) {
                 $extra = $list->getExtraFields();
 
                 if ($extra && array_key_exists($col, $extra)) {