Add reserver to reservation creation log item

[Currently](https://github.com/YaleSTC/reservations/blob/master/app/models/cart.rb#L89), our creation note looks like:

```
### Reserved on #{Time.zone.now.to_s(:long)} by #{user.md_link}
```

It should look like:

```
### Reserved on #{Time.zone.now.to_s(:long)} by #{user.md_link} for #{reserver.md_link}
```

Pretty quick, adding to v5.4.0.
