Refactor click handlers on upload button.
Refactor click handlers on upload button.

closes #2028
- Instead of binding and unbinding the click event, bind the data to the
  outer scope so we can use a standard click handler.
- Use removeProp instead of setting the property to false when enabling.
- Use the `disabled` as value when disabling.

kevinansfield/Ghost-Admin,kevinansfield/Ghost-Admin,airycanon/Ghost-Admin,acburdine/Ghost-Admin,dbalders/Ghost-Admin,dbalders/Ghost-Admin,TryGhost/Ghost-Admin,acburdine/Ghost-Admin,JohnONolan/Ghost-Admin,JohnONolan/Ghost-Admin,TryGhost/Ghost-Admin,airycanon/Ghost-Admin