Don't populate the password field on signup validation errors.

- Previously, we were pulling `params[:user][:password] as the default
  value for the password field. This is incorrect; we should be pulling
  it from `@user.password` or the like.

app/views/devise/shared/_signup_box.html.haml

@@ -17,7 +17,7 @@
       %div
         = f.email_field :email, class: "form-control middle", value: user[:email], placeholder: "Email", required: true
       .form-group.append-bottom-20#password-strength
-        = f.password_field :password, class: "form-control bottom", value: user[:password], id: "user_password_sign_up", placeholder: "Password", required: true
+        = f.password_field :password, class: "form-control bottom", id: "user_password_sign_up", placeholder: "Password", required: true
       %div
       - if current_application_settings.recaptcha_enabled
         = recaptcha_tags