Show warning dialog if session has ended in editor

Closes #3839
* Display a warning modal in the editor routes if the user
  is logged out due to re-authorization problems.  This provides
  a chance to copy any unsaved content before being sent to the
  signin page to log back in.

![publish-button](https://cloud.githubusercontent.com/assets/214142/4024349/e83ad824-2bc4-11e4-8b68-edfb71bf1523.gif)