Fix #4425: Add Danbooru Discord server prominently

* Add Discord, Twitter, and Github links to site footer.
* Remove "Desktop mode" and "Keyboard shortcuts" links from footer.
  Users can use the desktop site option in their browser to switch
  between desktop and mobile mode.
* Remove git hash from footer. Users can hover over the Github logo to
  see the hash instead.

app/javascript/src/javascripts/common.js

@@ -1,5 +1,4 @@
 import Cookie from './cookie'
-import CurrentUser from './current_user'
 
 $(function() {
   $("#hide-upgrade-account-notice").on("click.danbooru", function(e) {
@@ -20,12 +19,6 @@ $(function() {
     $('#notice').fadeOut("fast");
     e.preventDefault();
   });
-
-  $("#desktop-version-link a").on("click.danbooru", async function(e) {
-    e.preventDefault();
-    await CurrentUser.update({ enable_desktop_mode: true });
-    location.reload();
-  });
 });
 
 window.submitInvisibleRecaptchaForm = function () {