support: Revise management forms for deactivated realms.

The only form we want to show in the "Realm management" section of
the support view for deactivate realms is the one to reactivate it.

templates/corporate/support/realm_details.html

@@ -83,6 +83,7 @@
             </select>
             <button type="submit" class="support-submit-button">Update</button>
         </form>
+        {% if not realm.deactivated %}
         <form method="POST" class="support-form">
             <b>New subdomain</b>:<br />
             {{ csrf_input }}
@@ -131,6 +132,7 @@
             <input type="number" name="max_invites" value="{{ realm.max_invites }}" min="0" required />
             <button type="submit" class="support-submit-button">Update</button>
         </form>
+        {% endif %}
     </div>
     {% if realm.deactivated %}
     <div class="support-sponsorship-container">