Create a go to mapping modal

* When pressing “g m” a modal appears asking for an old URL, entering the URL will take you directly to that mappings edit screen
* “g m” for “go to mapping”
* Uses the existing find_global method to resolve a mapping without site

![screen shot 2014-04-11 at 13 44 34](https://cloud.githubusercontent.com/assets/319055/2680470/1c655e28-c183-11e3-888c-0c32e70b2de7.png)