Add drag and drop config import closes #11004

As requested by @vincent3569 this PR allows user to import config.json settings via a drag and drop import. Fixes #11004.
Attached are some screenshots of the UI changes.

Drop target
![screen shot 2014-06-11 at 10 49 26](https://cloud.githubusercontent.com/assets/678372/3242850/56e89090-f150-11e3-8808-23f6bd7dd71b.jpg)

Error on uploading invalid file
![screen shot 2014-06-11 at 10 49 47](https://cloud.githubusercontent.com/assets/678372/3242852/58e3b532-f150-11e3-854c-3152b2db44b6.jpg)

Success message
![screen shot 2014-06-11 at 11 04 36](https://cloud.githubusercontent.com/assets/678372/3242853/5bcae874-f150-11e3-9063-153df42be53b.jpg)

Side nav
![screen shot 2014-06-11 at 11 04 53](https://cloud.githubusercontent.com/assets/678372/3242855/5ed4bb1c-f150-11e3-97bb-808aa150d4c1.jpg)

Semicolon and you could define this variable at the very top of the file, as you don't only use this here but also earlier when you attach the listeners for the `dragover` and `drop` event.