Display title in nav only if PLATFORM_NAME is not set

Why these changes are being introduced:

The app name is inserted into the header based on the `PLATFORM_NAME`
environment variable. If this variable is set, the title should
not display

Relevant ticket(s):

https://mitlibraries.atlassian.net/browse/GDT-124

How this addresses that need:

This makes the title in the nav render if `PLATFORM_NAME` is not
set.

Side effects of this change:

We will need to make this change downstream, as most (if not all)
of our Rails properties have customized the site nav partial.

Author: jazairi

app/views/layouts/_site_nav.html.erb

@@ -1,8 +1,10 @@
 <div class="wrap-outer-header-local layout-band">
   <div class="wrap-header-local">
-    <div class="local-identity">
-      <h2 class="title title-site"><a href="/">Site Title</a></h2>
-    </div>
+    <%= unless ENV['PLATFORM_NAME'] %>
+      <div class="local-identity">
+        <h2 class="title title-site"><a href="/">Site Title</a></h2>
+      </div>
+    <% end %>
     <div class="wrap-local-nav">
       <div class="wrap-bar">
         <nav class="local-nav" aria-label="Main menu">