File transfer improvement

This PR improve a few things about the file transfers:

- The transfers are now completely synchronized between the sender and the recipient
- The transfers now have a progress bar and are closer to the mockups [1]

However I think it still needs some CSS work.

[1] https://trello.com/c/0BT8aoGV/187-2-as-a-user-i-know-the-status-of-a-content-transfer