Add alt tags to menu buttons for screen readers

// FREEBIE

background.html

@@ -8,7 +8,7 @@
           <div class='header-buttons right'>
             <div class='vertical-align'>
               <div class='global-menu menu'>
-                <button class='hamburger'></button>
+                <button class='hamburger' alt='signal menu'></button>
                 <ul class='menu-list'>
                     <li class='settings'>{{ settings }}</li>
                     <li class='show-debug-log'>{{ submitDebugLog }}</li>
@@ -61,7 +61,7 @@
         <div class='header-buttons right'>
           <div class='vertical-align'>
             <div class='conversation-menu menu'>
-              <button class='hamburger'></button>
+              <button class='hamburger' alt='conversation menu'></button>
               <ul class='menu-list'>
                   {{#group}}
                       <li class='view-members'>{{ view-members }}</li>