Disable filter textbox until pageload.

For channels with many thousands of videos, it can be a while
before the DOM is fully parsed and ready to go. It is annoying
to type in the box and find that it doesn't work yet, since the
script tag comes after body.
Why not move the script above body? Because I don't want to,
and because you'd be filtering from an incomplete list of
videos anyway.

frontends/ycdl_flask/templates/channel.html

@@ -236,7 +236,7 @@ https://stackoverflow.com/a/35153397
 
 
     <div id="video_cards">
-        <center><input type="text" id="search_filter"/></center>
+        <center><input disabled class="enable_on_pageload" type="text" id="search_filter"/></center>
         <center><span id="search_filter_count">{{videos|length}}</span> items</center>
 
         {% for video in videos %}