Multiple front-end fixes (#2247)

Fixes:
* Sanitize user-provided content in HTML (Fixes #2193)
* Fix encoding of search query in prev/next pages (Fixes #2229)
* Fix some issues introduced with #2196:
   - Fix alignment of all <h3> elements (Move the inline style from the parent to the <h3> element)
   - Add missing comma on 'dir' HTML attribute (Typo introduced by PR #2196)

Code cleaning:
* Remove unnecessary 'each_sclice' + 'each' double loop in ECR files
* Clean the player's <source> list generation code (in player.ecr)

src/invidious/views/mix.ecr

@@ -1,22 +1,20 @@
 <% content_for "header" do %>
-<title><%= mix.title %> - Invidious</title>
+<title><%= HTML.escape(mix.title) %> - Invidious</title>
 <% end %>
 
 <div class="pure-g h-box">
     <div class="pure-u-2-3">
-        <h3><%= mix.title %></h3>
+        <h3><%= HTML.escape(mix.title) %></h3>
     </div>
-    <div class="pure-u-1-3" style="text-align:right">
-        <h3>
+    <div class="pure-u-1-3">
+        <h3 style="text-align:right">
             <a href="/feed/playlist/<%= mix.id %>"><i class="icon ion-logo-rss"></i></a>
         </h3>
     </div>
 </div>
 
 <div class="pure-g">
-    <% mix.videos.each_slice(4) do |slice| %>
-        <% slice.each do |item| %>
-            <%= rendered "components/item" %>
-        <% end %>
-    <% end %>
+<% mix.videos.each do |item| %>
+    <%= rendered "components/item" %>
+<% end %>
 </div>