MDL-58248 theme_boost: label/span for sidepanel expand button

Add string "Side panel" to make the purpose of
trigram/hamburger icon at top left clearer, particularly
when using screen readers e.g. Jaws/NVDA.
Currently it reads out only "expand"

lang/en/moodle.php

@@ -1741,6 +1741,7 @@
 $string['showtheselogs'] = 'Show these logs';
 $string['showthishelpinlanguage'] = 'Show this help in language: {$a}';
 $string['schedule'] = 'Schedule';
+$string['sidepanel'] = 'Side panel';
 $string['signoutofotherservices'] = 'Sign out everywhere';
 $string['signoutofotherservices_help'] = 'If ticked, the account will be signed out of all devices and systems which use web services, such as the mobile app.';
 $string['since'] = 'Since';

theme/boost/templates/header.mustache

@@ -22,7 +22,7 @@
     <div class="container-fluid navbar-nav">
 
         <div data-region="drawer-toggle">
-            <button aria-expanded="{{#navdraweropen}}true{{/navdraweropen}}{{^navdraweropen}}false{{/navdraweropen}}" aria-controls="nav-drawer" type="button" class="btn pull-xs-left m-r-1 btn-secondary" data-action="toggle-drawer" data-side="left" data-preference="drawer-open-nav"><span aria-hidden="true">&#9776;</span><span class="sr-only">{{#str}}expand, core{{/str}}</span></button>
+            <button aria-expanded="{{#navdraweropen}}true{{/navdraweropen}}{{^navdraweropen}}false{{/navdraweropen}}" aria-controls="nav-drawer" type="button" class="btn pull-xs-left m-r-1 btn-secondary" data-action="toggle-drawer" data-side="left" data-preference="drawer-open-nav"><span aria-hidden="true">&#9776;</span><span class="sr-only">{{#str}}sidepanel, core{{/str}}</span></button>
         </div>
 
         <a href="{{{ config.wwwroot }}}" class="navbar-brand {{# output.should_display_navbar_logo }}has-logo{{/ output.should_display_navbar_logo }}