#13 Make login work w/ enter key

[Matt Palmerlee]

Modify the way in which the login mechanism works so
that users can hit enter after entering their password
instead of having to click the login button (or tab onto it
and then click enter).
I think that this means that the button must be a submit
button instead of a button with an OnClick handler.

As part of this change, the new method of using a
submit button needs to be made secure (someone
hitting the back button can't resubmit the username and
password to login).