portico: Set standalone 'Choose a plan' header for self-hosters.

templates/corporate/pricing_model.html

@@ -1,5 +1,8 @@
 <div id="plans-and-pricing" class="pricing-model">
     <div class="padded-content">
+        {% if is_self_hosted_realm %}
+        <h1>Choose a plan</h1>
+        {% else %}
         <h1>Zulip plans and pricing</h1>
         <div class="h1-subheader">
             You can <a href="https://zulip.com/help/zulip-cloud-or-self-hosting">move
@@ -9,6 +12,7 @@
             <a href="https://zulip.readthedocs.io/en/stable/production/export-and-import.html">import
             </a> tools.
         </div>
+        {% endif %}
         <div class="pricing-container">
             <div id="cloud" class="cloud-plan-title pricing-tab">
                 <h2>Zulip Cloud</h2>