Improve the UX of closing the editor

[oanalavinia]

The editor can be closed by clicking on one of the 3 buttons / links from the upper menu: link to the home page, link to the file's page and cancel button.
Each of these should notify the user if there are unsaved changes that will be lost.

Also, the cancel button suggests that it might cancel any editing / saving done until now, while all it does is a close without save, so the label could be improved.