Clean up merging stuff

src copy/Core/Support/MergeableInterface.php

@@ -0,0 +1,6 @@
+<?php namespace Flarum\Core\Models;
+
+interface MergeableInterface
+{
+    public function saveAfter(Model $previous);
+}

src copy/Core/Support/MergeableTrait.php

@@ -0,0 +1,23 @@
+<?php namespace Flarum\Core\Models;
+
+trait MergeableTrait
+{
+    public function saveAfter(Model $previous)
+    {
+        if ($previous instanceof static) {
+            if ($merged = $this->mergeInto($previous)) {
+                $merged->save();
+                return $merged;
+            } else {
+                $previous->delete();
+                return $previous;
+            }
+        }
+
+        $this->save();
+
+        return $this;
+    }
+
+    abstract protected function mergeInto(Model $previous);
+}