Added multiline "placeholder" to 3rd step of setup

Closes #5459
- added a PNG background with 2 emails addresses as a "placeholder"
- added a `required` attribute for textarea in 3rd step of the setup
- taking an advantage of `:valid` pseudo-class to hide the background
  picture with email addresses

app/styles/layouts/flow.css

@@ -384,6 +384,16 @@
     line-height: 1.4em;
 }
 
+.gh-flow-invite textarea {
+    background: url(img/invite-placeholder.png) 8px 10px no-repeat;
+    background-size: 202px 48px;
+    box-shadow: none; /* Remove some default styling for Firefox (required attribute) */
+}
+
+.gh-flow-invite textarea:valid {
+    background: none;
+}
+
 @media (max-width: 460px) {
     .gh-flow-content .gh-flow-invite label i {
         display: none;

app/templates/setup/three.hbs

@@ -8,7 +8,7 @@
 
     <form class="gh-flow-invite">
         <label>Enter one email address per line, we’ll handle the rest! <i class="icon-mail"></i></label>
-        {{textarea class="gh-input" name="users" placeholder="john@example.com" value=users}}
+        {{textarea class="gh-input" name="users" value=users required="required"}}
     </form>
 
     <button {{action 'invite'}} class="btn btn-default btn-lg btn-block {{buttonClass}}">