Update WebUI to be more compliant with HTML5 standard

Use modern and shorter "charset" attribute.
Remove "type", "charset" attributes in script/style tags, these are redundant in HTML5.

src/webui/www/private/preferences_content.html

@@ -479,7 +479,7 @@
 <br/>
 <div style="text-align: center;"><input type="button" value="QBT_TR(Save)QBT_TR[CONTEXT=HttpServer]" onclick="applyPreferences();"/></div>
 
-<script type="text/javascript">
+<script>
 // Downloads tab
 var WatchedFoldersTable = new HtmlTable($("watched_folders_tab"));