Implemented phase 2 for the POC prototype in the Website service.

Services/Website/Sources/Library/Internal/Components/SiteHeader.swift

@@ -218,12 +218,25 @@ private extension SiteHeader {
             )
         ) {
             span(.class("from-desktop")) {
-                viewer.displayName
+                viewer.firstName
             }
             span(.class("site-header__avatar")) {
                 viewer.initials
             }
         }
+        // Not in the design, whose sign-out lives on the account screen the prototype leaves out.
+        form(
+            .class("site-header__sign-out"),
+            .action(.Route.signOut),
+            .method(.post)
+        ) {
+            button(
+                .class("site-header__text-link"),
+                .type(.submit)
+            ) {
+                "Sign out"
+            }
+        }
     }
 
     /// An icon link with a count badge, omitted at zero.