use mongoose transaction helper

use the helper instead of manually commiting/aborting
to deal with transient transaction errors

website/server/controllers/api-v3/groups.js

@@ -138,19 +138,11 @@ api.createGroup = {
     }
 
     let savedGroup;
-    const session = await Group.startSession();
 
-    try {
-      session.startTransaction();
+    await Group.db.transaction(async session => {
       await user.save({ session });
       savedGroup = await group.save({ session });
-      await session.commitTransaction();
-    } catch (error) {
-      await session.abortTransaction();
-      throw error;
-    } finally {
-      session.endSession();
-    }
+    });
 
     // Instead of populate we make a find call manually because of https://github.com/Automattic/mongoose/issues/3833
     // await Q.ninvoke(savedGroup, 'populate', ['leader', nameFields]);