refactor copy into model

tracim/tracim/lib/content.py

         else:
             label = item.label
 
-        content = Content()
-        # INFO - G.M - 15-03-2018 - Add all revisions (history)
-        for rev in item.revisions:
-            cpy_rev = ContentRevisionRO.copy(rev, parent)
-            content.revisions.append(cpy_rev)
-
+        content = item.copy(parent)
         # INFO - GM - 15-03-2018 - add "copy" revision
         content.new_revision()
         content.parent = parent

tracim/tracim/model/data.py

         cid = content.content_id
         return url_template.format(wid=wid, fid=fid, ctype=ctype, cid=cid)
 
+    def copy(self, parent):
+        cpy_content = Content()
+        for rev in self.revisions:
+            cpy_rev = ContentRevisionRO.copy(rev, parent)
+            cpy_content.revisions.append(cpy_rev)
+        return cpy_content
+
 
 class RevisionReadStatus(DeclarativeBase):