Start work on new sign-in screen

WMT needs a real sign-in screen to prevent users from accessing WMT
functionality without a login. This screen will be displayed on
startup. Currently, the widgets are set up, but no event handlers have
been hooked up.
Add handlers for question links to sign-in screen

The "New User?" and "Forgot Password?" questions on the sign-in screen
now have ClickHandlers that display information.

Also changed vertical alignment of content: instead of center-aligned,
it's now top-aligned with an offset calculated from the browser window
height.

The width of the content is also constrained by the browser window
width.
Add CSDMS Support email to "Forgot Password?" link
Move question link text to Constants class
Experiment with showing WMT GUI after sign-in

Remove the sign-in screen from the RootLayoutPanel, then add the
Perspective. The Perspective has been loading in the background while
the sign-in screen was showing. Neat!

The remove/add is done in SignInScreen. This will likely be moved to
AuthneticationHandler.
Add getter/setter for SignInScreen to DataManager

This allows it to be swapped with the Perspective in the
RootLayoutPanel.
On successful login, show the Perspective

Remove the SignInScreen and add the Perspective to the RootLayoutPanel.
Make a signIn method for AuthenticationHandler

The old onClick method was kludgy. I'll remove onClick when I tackle
the sign out action.
Add AuthenticationHandler to SignInScreen

Call its signIn method when the user clicks the "Sign In" button, or
when the user hits <Enter> in the password box.
Renamed LoginPanel to UserPanel

Also stripped away all login actions.
Deprecate DesensitizingPopupPanel

It's no longer needed with the new SignInScreen.
Add getter/setter for UserPanel#signInButton
Update CSS with new rule for UserPanel
Add sign out handler to UserPanel on Perspective
Store the AuthenticationHandler in Security

It can be conveniently accessed from the DataManager object.
Clean up AuthenticationHandler

Remove its ClickHandler.
Temporarily disable check for user already signed in

Though I'm thinking about making this permanent. You close your WMT
window, you have to sign in again.