Update theme gem and add skip_links partial

Why these changes are being introduced:

We are trying out custom skip links for this application, as
it has a relatively complex navigation structure. The theme gem
has been updated to support this, using a new `_skip_links` partial.

Relevant ticket(s):

* [GDT-161](https://mitlibraries.atlassian.net/browse/GDT-161)

How this addresses that need:

This updates the theme gem and adds the new partial as-is. It will
be customized in a subsequent commit.

Side effects of this change:

A small amount of technical debt may be added here, but incorporating
a layout that needs to remain compatible with the upstream theme
gem. However, the partial itself is very simple, so it's unlikely
that any maintenance will be required.

Author: jazairi

Gemfile

@@ -9,7 +9,7 @@ gem 'graphql-client'
 gem 'http'
 gem 'importmap-rails'
 gem 'jbuilder'
-gem 'mitlibraries-theme', git: 'https://github.com/mitlibraries/mitlibraries-theme', tag: 'v1.2'
+gem 'mitlibraries-theme', git: 'https://github.com/mitlibraries/mitlibraries-theme', tag: 'v1.4'
 gem 'puma'
 gem 'rails', '~> 7.0'
 gem 'sentry-rails'

Gemfile.lock

@@ -1,7 +1,7 @@
 GIT
   remote: https://github.com/mitlibraries/mitlibraries-theme
-  revision: bcbe5d3de36a92d275085a045c5c4d8f30f33e62
-  tag: v1.2
+  revision: ba5bdc9840ef817ba0b92e522b92da3f52669fc3
+  tag: v1.4
   specs:
     mitlibraries-theme (1.0.2)
       rails (>= 6, < 8)

app/views/layouts/_skip_links.html.erb

@@ -0,0 +1 @@
+<a id="skip" class="sr sr-focusable" href="#content-main">Skip to main content</a>