Fix #96: Add a new Layout for the Admin Area.

I've included a start of a new layout for the Admin Area, but it will need more work in future.

This fixes the immediate issue, but there's a problem with the Create Session link in the admin area: its using the Admin Route and so 404s. We can at least access the admin pages again.

**Note:** This builds on #107, so **do not merge** until that has gone in.