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/subscription_manager.ecr

@@ -10,15 +10,15 @@
             </a>
         </h3>
     </div>
-    <div class="pure-u-1-3" style="text-align:center">
-        <h3>
+    <div class="pure-u-1-3">
+        <h3 style="text-align:center">
             <a href="/feed/history">
                 <%= translate(locale, "Watch history") %>
             </a>
         </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="/data_control?referer=<%= URI.encode_www_form(referer) %>">
                 <%= translate(locale, "Import/export") %>
             </a>
@@ -31,7 +31,7 @@
         <div class="pure-g<% if channel.deleted %> deleted <% end %>">
             <div class="pure-u-2-5">
                 <h3 style="padding-left:0.5em">
-                    <a href="/channel/<%= channel.id %>"><%= channel.author %></a>
+                    <a href="/channel/<%= channel.id %>"><%= HTML.escape(channel.author) %></a>
                 </h3>
             </div>
             <div class="pure-u-2-5"></div>