signup: point to /register at the top too.

The message to use /register if your group is already signed up is
below the fold on my laptop -- this probably contributes to people
trying to sign up their group when they are just trying to register.

(imported from commit 517df29d10182c05780017c15225c9f95b844225)

static/styles/portico.css

@@ -262,7 +262,7 @@ img.screenshot{
     margin: auto;
     margin-bottom: 20px;
     text-align: center;
-    margin-top: 30px;
+    margin-top: 10px;
 }
 
 .portico-page h1 {
@@ -610,13 +610,17 @@ a.bottom-signup-button {
     margin-bottom: -54px;
 }
 
-.signup-page-bottom {
+.signup-page-bottom, .signup-page-top {
     width: 100%;
     max-width: 600px;
     text-align: center;
     margin: auto;
 }
 
+.signup-page-top {
+    margin-top: 10px;
+}
+
 .signup-signature {
     margin-top: 20px;
     padding-bottom: 50px;

templates/corporate/signup.html

@@ -8,6 +8,11 @@
 {% endblock %}
 
 {% block portico_content %}
+
+    <div class="signup-page-top">
+      Does your company already use Zulip? <a href="{% url 'register' %}">Register your account here</a>.
+    </div>
+
     <div class="signup-header">
       <h1>Sign up now!</h1>
       <h4>Zulip is currently in closed beta,<br />