Documents: Organize methods per best practices

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/documents/models.py

@@ -415,14 +415,6 @@ class DocumentVersion(models.Model):
     _pre_open_hooks = {}
     _post_save_hooks = {}
 
-    @classmethod
-    def register_pre_open_hook(cls, order, func):
-        cls._pre_open_hooks[order] = func
-
-    @classmethod
-    def register_post_save_hook(cls, order, func):
-        cls._post_save_hooks[order] = func
-
     document = models.ForeignKey(
         on_delete=models.CASCADE, related_name='versions', to=Document,
         verbose_name=_('Document')
@@ -475,6 +467,14 @@ class DocumentVersion(models.Model):
     def __str__(self):
         return self.get_rendered_string()
 
+    @classmethod
+    def register_pre_open_hook(cls, order, func):
+        cls._pre_open_hooks[order] = func
+
+    @classmethod
+    def register_post_save_hook(cls, order, func):
+        cls._post_save_hooks[order] = func
+
     @cached_property
     def cache(self):
         Cache = apps.get_model(app_label='common', model_name='Cache')