Fix channel sort on mobile

src/invidious/views/channel.ecr

@@ -49,7 +49,7 @@
     <div class="pure-u-1-3">
         <div class="pure-g" style="text-align:right;">
         <% {"newest", "oldest", "popular"}.each do |sort| %>
-            <div class="pure-u-1-3">
+            <div class="pure-u-1 pure-md-1-3">
                 <% if sort_by == sort %>
                 <b><%= sort %></b>
                 <% else %>