adds an accessible label to the search input and the search button

lms/templates/index.html

@@ -39,9 +39,11 @@ from microsite_configuration import microsite
         % if settings.FEATURES.get('ENABLE_COURSE_DISCOVERY'):
           <div class="course-search">
             <form method="get" action="/courses">
-              <input class="search-input" name="search_query" type="text" placeholder="${_("Search for a course")}"></input>
+              <label><span class="sr">${_("Search for a course")}</span>
+                <input class="search-input" name="search_query" type="text" placeholder="${_("Search for a course")}"></input>
+              </label>
               <button class="search-button" type="submit">
-                <i class="icon fa fa-search" aria-hidden="true"></i>
+                <i class="icon fa fa-search" aria-hidden="true"></i><span class="sr">${_("Search")}</span>
               </button>
             </form>
           </div>