fix missing login button due to layout refactoring

src/app/shared/layout/layout.component.html

 <div id="layout">
-  <app-navbar [links]="links" [theme]="theme"></app-navbar>
+  <app-navbar [links]="links" [theme]="theme">
+    <ng-content app-navbar-extra select="[app-navbar-extra]"></ng-content>
+    <ng-content app-navbar-extra-narrow select="[app-navbar-extra-narrow]"></ng-content>
+  </app-navbar>
   <main class="content">
    <router-outlet (deactivate)="onDeactivate()"></router-outlet>
   </main>

src/app/showcase-site/showcase-site.component.html

-<app-layout [links]="navLinks"></app-layout>
+<app-layout [links]="navLinks">
+  <app-login-button app-navbar-extra class="login-link"></app-login-button>
+  <app-login-button app-navbar-extra-narrow class="text-center login-link"></app-login-button>
+</app-layout>

src/app/showcase-site/showcase-site.component.scss

+.login-link {
+  display: block;
+  margin: auto 0;
+  padding: 1em 0;
+}