Fix Sync button tab navigation

Fixes https://github.com/mozilla/notes/issues/495

src/sidebar/panel.html

@@ -31,11 +31,11 @@
     <div id="footer-buttons" class="savingLayout">
       <div>
         <span id="saving-indicator"></span>
-        <a id="enable-sync" class="notsyncing">
+        <button id="enable-sync" class="notsyncing">
           <svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 16 16">
             <path fill="context-fill" d="M14 1a1 1 0 0 0-1 1v1.146A6.948 6.948 0 0 0 1.227 6.307a1 1 0 1 0 1.94.484A4.983 4.983 0 0 1 8 3a4.919 4.919 0 0 1 3.967 2H10a1 1 0 0 0 0 2h4a1 1 0 0 0 1-1V2a1 1 0 0 0-1-1zm.046 7.481a1 1 0 0 0-1.213.728A4.983 4.983 0 0 1 8 13a4.919 4.919 0 0 1-3.967-2H6a1 1 0 0 0 0-2H2a1 1 0 0 0-1 1v4a1 1 0 0 0 2 0v-1.146a6.948 6.948 0 0 0 11.773-3.161 1 1 0 0 0-.727-1.212z"/>
           </svg>
-        </a>
+        </button>
       </div>
       <a id="give-feedback-button">
         <svg id="Layer_1" width="20" height="20" data-name="Layer 1" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16">