twig: update social links to have sr-only text for accessiblity

src/twig/partials/social-links.twig

@@ -1,12 +1,23 @@
 <ul class="d-inline-flex {{class}}">
-  {% for name in ['linkedin', 'instagram', 'facebook'] %}
+  {% for name in [
+    { name: 'linkedin',
+      text: "Go to Middlebury's LinkedIn page"
+    }, 
+    { name: 'instagram',
+      text: "Go to Middlebury's Alumni page on Instagram"
+    }, 
+    { name: 'facebook',
+      text: "Go to Middlebury's Alumni page on Facebook"
+    }] 
+  %}
     {# {% if env.test or random(1) %} #}
       <li class="mr-1">
         <a href="#" class="d-inline-block {% if light %}text-white hover-fadeout{% else %}hover-fadein{% endif %} {{office_footer ? " px-1" : " p-1"}}">
           {% include 'partials/icon.twig' with {
-            name: name,
+            name: name.name,
             class: large ? 'f5' : null
           } %}
+          <span class="sr-only">{{name.text}}</span>
         </a>
       </li>
     {# {% endif %} #}