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/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 {