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 () {

app/javascript/src/styles/base/020_base.scss

@@ -126,7 +126,7 @@ table tfoot {
 }
 
 .text-small {
-  font-size: 0.8em;
+  font-size: 0.9em;
 }
 
 .fixed-width-container {

app/javascript/src/styles/common/main_layout.scss

@@ -30,6 +30,12 @@ footer#page-footer {
   text-align: center;
   padding: 1em 0 1em;
   border-top: var(--footer-border);
+
+  .social-icon img {
+    vertical-align: bottom;
+    margin: 0 0.1em;
+    height: 16px;
+  }
 }
 
 .sidebar-container {

app/views/static/_footer.html.erb

@@ -1,21 +1,23 @@
 <footer id="page-footer" class="text-small">
   <span class="page-footer-app-name"><%= Danbooru.config.app_name %></span>
-  <% if Rails.application.config.x.git_hash %>
-    <span class="desktop-only page-footer-git-hash">
-      (<%= link_to Rails.application.config.x.git_hash.first(9), Danbooru.config.commit_url(Rails.application.config.x.git_hash) %>)
-    </span>
-  <% end %>
-  &ndash; <%= link_to "Rules", terms_of_service_path %>
-  &ndash; <%= link_to "Contact", contact_path %>
-  <% if CurrentUser.user.enable_post_navigation %>
-    <span id="keyboard-shortcuts-link" class="desktop-only">
-      &ndash; <%= link_to "Keyboard shortcuts", keyboard_shortcuts_path %>
-    </span>
-  <% end %>
-  <% if CurrentUser.is_member? %>
-    <span id="desktop-version-link" class="mobile-only">
-      &ndash;
-      <%= link_to "Desktop mode", edit_user_path(CurrentUser.id) %>
-    </span>
-  <% end %>
+  / <%= link_to "Rules", terms_of_service_path %>
+  / <%= link_to "Contact", contact_path %>
+  /
+  <span class="social-icons">
+    <% if Danbooru.config.source_code_url.present? %>
+      <%= link_to Danbooru.config.source_code_url, title: "Running commit: #{Rails.application.config.x.git_hash.first(9)}", class: "social-icon" do %>
+        <img src="images/github-logo.png">
+      <% end %>
+    <% end %>
+    <% if Danbooru.config.twitter_username.present? %>
+      <%= link_to "https://twitter.com/#{Danbooru.config.twitter_username}", class: "social-icon" do %>
+        <img src="images/twitter-logo.png">
+      <% end %>
+    <% end %>
+    <% if Danbooru.config.discord_server_url.present? %>
+      <%= link_to Danbooru.config.discord_server_url, class: "social-icon" do %>
+        <img src="images/discord-logo.png">
+      <% end %>
+    <% end %>
+  </span>
 </footer>

config/danbooru_default_config.rb

@@ -425,6 +425,11 @@ module Danbooru
       end
     end
 
+    # The url of the Discord server associated with this site.
+    def discord_server_url
+      nil
+    end
+
     # The twitter username associated with this site (username only, don't include the @-sign).
     def twitter_username
       nil