Update theme gem and document PLATFORM_NAME variable

Why these changes are being introduced:

The GDT project added a feature to the theme gem to conditionally
include the platform name in the header if the `PLATFORM_NAME`
environment variable is supplied.

Relevant ticket(s):

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

How this addresses that need:

This updates the theme gem to the v1.2 tag, which includes the new
feature, and documents the PLATFORM_NAME variable. It also hides
the title in the navbar if PLATFORM_NAME is present.

Side effects of this change:

None. The app will continue to use the standard header if
PLATFORM_NAME is not supplied.

Author: jazairi

Gemfile

@@ -10,7 +10,7 @@ gem 'flipflop'
 gem 'graphql'
 gem 'jwt'
 gem 'lograge'
-gem 'mitlibraries-theme', git: 'https://github.com/mitlibraries/mitlibraries-theme', tag: 'v1.1'
+gem 'mitlibraries-theme', git: 'https://github.com/mitlibraries/mitlibraries-theme', tag: 'v1.2'
 gem 'opensearch-ruby'
 gem 'puma'
 gem 'rack-attack'

Gemfile.lock

@@ -1,7 +1,7 @@
 GIT
   remote: https://github.com/mitlibraries/mitlibraries-theme
-  revision: 93b931b802485f9e35a6878f957b3fd88ae3b294
-  tag: v1.1
+  revision: bcbe5d3de36a92d275085a045c5c4d8f30f33e62
+  tag: v1.2
   specs:
     mitlibraries-theme (1.0.2)
       rails (>= 6, < 8)

README.md

@@ -130,6 +130,7 @@ locally.
   confused.
   ```
 
+- `PLATFORM_NAME`: The value set is added to the header after the MIT Libraries logo. The logic and CSS for this comes from our theme gem.
 - `PREFERRED_DOMAIN` - set this to the domain you would like to to use. Any
   other requests that come to the app will redirect to the root of this domain.
   This is useful to prevent access to herokuapp.com domains.

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="/">TIMDEX</a></h2>
-    </div>
+    <% unless ENV['PLATFORM_NAME'] %>
+      <div class="local-identity">
+        <h2 class="title title-site"><a href="/">TIMDEX</a></h2>
+      </div>
+    <% end %>
     <div class="wrap-local-nav">
       <div class="wrap-bar">
         <nav class="local-nav" aria-label="Main menu">