Disable window close event in sign-in screen

We've been trapping the window close event and reminding users that
info will be lost if the leave the page. I've changed this so the
handler is now associated with the Perspective; it's attached when the
user signs in, and removed when the user signs out. It won't trap a
refresh/close of the sign-in page.
Add new Constant for CSDMS email
Show sign-in errors on the SignInScreen

Previously, they were displayed in pop-up windows.
On logout, clear password box in SignInScreen

Even though the password is obscured, it shouldn't be there. Let the
user type it in again.
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