Open the codebase dialog when returning from GitHub

Coming back from GitHub sign-in, cancellation or app installation now
opens the Open codebase dialog automatically with the outcome at the
top (signed in as whoever, access updated, cancelled or failed). A
repository the visitor was trying to open is filled in with Explore
focused, so it is one keypress to continue, instead of loading it
silently in the background. The ?github= parameter is removed from the
address bar and any shared-place fragment is kept.

web/index.html

@@ -136,6 +136,7 @@
         <div><small>YOUR NEXT PERSPECTIVE</small><h1 id="loadTitle">Open a codebase</h1><p class="dialog-description">See the big picture. Explore every detail.</p></div>
         <button class="close-button" value="cancel" aria-label="Close"><svg viewBox="0 0 24 24" aria-hidden="true"><path d="m6 6 12 12M18 6 6 18"/></svg></button>
       </div>
+      <p class="github-notice" id="githubNotice" role="status" hidden></p>
       <button type="button" class="source-card" id="localFolderButton">
         <span class="source-icon"><svg viewBox="0 0 24 24" aria-hidden="true"><path d="M3 7h7l2-2h4l2 2h3v12H3zM3 10h18"/></svg></span>
         <span><strong>Local folder</strong><small>Temporarily indexed by this server</small></span>