API Enable reauthentication within the CMS if a user session is lost

This PR provides an in-cms popup dialog allowing users to re-authenticate themselves if their session is lost (e.g. they logged out in another tab).

![screen shot 2014-10-10 at 11 35 10 am](https://cloud.githubusercontent.com/assets/936064/4585346/a861f19c-5004-11e4-89b1-4d53bb9e2905.png)

The updated assets / styles seem to be caused by compass regenerating the files.

Existing custom authenticators will need to expliictly enable CMS support. By default this feature will only work with the MemberAuthenticator, and will be disabled if no supporting authenticators are setup (fall back to current system).

To support the communication of messages between frames I've had to implement a message event handler in LeftAndMain.js. I'd appreciate someone with javascript background taking a look at this to evaluate it from a security point of view.

The behaviour of default admin has changed slightly, in order to be a little more secure. This now no longer automatically logs you in as any admin, and now only operates on a separate default-admin-only database user.
Could we make this an `Exception` instead? I know it's not technically related to this work, but would be good to clean this up a bit.