Add 'Close' & 'Back' aria-label attributes to buttons. (#3161)

* Add aria-label attributes to close buttons in nav, welcome hero, modal close.
* Replace title with aria-label in nav back button.

locale/core.yml

@@ -497,6 +497,9 @@ core:
       resend_button: Resend Confirmation Email
       sent_message: Sent
 
+    welcome_hero:
+      hide: Hide welcome message
+
   # Translations in this namespace are used by the forum and admin interfaces.
   lib:
     debug_button: Debug
@@ -539,6 +542,10 @@ core:
     loading_indicator:
       accessible_label: => core.ref.loading
 
+    # These translations are used in modals.
+    modal:
+      close: Close
+
     # These translations are used in the navigation header.
     nav:
       drawer_button: Open Navigation Drawer