Add authentication via devise

Signed-off-by: Tobias Eidelpes <tobias@eidelpes.info>

app/views/layouts/application.html.erb

@@ -18,6 +18,7 @@
       <label for="nav-toggle" class="burger-menu">
         <%= show_svg('menu-white-24dp.svg') %>
       </label>
+      <a href="#" class="logo">tiss2go</a>
       <div class="left-menu">
         <%= link_to 'Home', root_path %>
         <%= link_to 'People', people_index_path %>
@@ -25,16 +26,23 @@
         <%= link_to 'Projects', projects_index_path %>
         <%= link_to 'Theses', theses_index_path %>
       </div>
-      <a href="#" class="logo">tiss2go</a>
       <div class="right-menu">
-        <!-- TODO: Change root_path to login/register page -->
-        <%= link_to 'Logout', root_path %>
+        <% if user_signed_in? %>
+          <%= link_to 'Edit profile', edit_user_registration_path %><!--
+          --><%= link_to "Logout", destroy_user_session_path, method: :delete %>
+        <% else %>
+          <%= link_to "Sign up", new_user_registration_path %><!--
+          --><%= link_to "Login", new_user_session_path %>
+        <% end %>
       </div>
     </nav>
 
-  <main>
-    <%= yield %>
-  </main>
+    <main>
+      <p class="notice"><%= notice %></p>
+      <p class="alert"><%= alert %></p>
+
+      <%= yield %>
+    </main>
 
   </body>
 </html>