Njc can edit dive site info

https://trello.com/c/yCcTykxK/29-users-can-add-dive-site-information

![editdivesites](https://cloud.githubusercontent.com/assets/8681076/5264293/95c5515a-7a07-11e4-886f-4267bffa9fac.gif)

ifs can be condensed to `if signed_in? && current_user.can_modify?`