css: add .mobile-only, .desktop-only classes.

app/views/layouts/default.html.erb

@@ -103,7 +103,7 @@
   <header id="top">
     <h1 id="app-name-header"><%= link_to Danbooru.config.app_name, "/" %></h1>
     
-    <div id="maintoggle">
+    <div id="maintoggle" class="mobile-only">
       <a href="#"><i id="maintoggle-on" class="fas fa-bars"></i></a>
       <a href="#"><i id="maintoggle-off" class="fas fa-times" style="display: none;"></i></a>
     </div>

app/views/posts/index.html.erb

@@ -36,7 +36,7 @@
       <li><%= link_to "Wiki", new_wiki_page_path(wiki_page: { title: @post_set.tag_string }), id: "show-excerpt-link" %></li>
     <% end %>
 
-    <li id="searchbox-redirect-link"><a href="#search-box">Search &raquo;</a></li>
+    <li id="searchbox-redirect-link" class="mobile-only"><a href="#search-box">Search &raquo;</a></li>
   </menu>
 
   <%= render "posts/partials/index/edit" %>

app/views/static/_footer.html.erb

@@ -6,12 +6,12 @@
   &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">
+    <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">
+    <span id="desktop-version-link" class="mobile-only">
       &ndash;
       <%= link_to "Disable responsive mode", edit_user_path(CurrentUser.id), :rel => "nofollow" %>
     </span>