Minor clean up and tuning

1. Modified _discuzcode_video_replace to pass $message by reference
   instead of pass by value. More memory efficient.

2. Removed obsoleted functions `_discuzcode_video_html_callback()`
   and `_discuzcode_video_script_url()`.

3. Minor spacing adjustment.

4. Change most `"` into `'`. Better memory usage.
Moved URL cleaning code back from Widgetfy

It should be this library's responsibility to provide a cleaned
URL to Widgetfy library.
Start using Widgetfy to handle media files
Migrated Ogg related code to Widgetfy
Migrated classic video related code to Widgetfy

Migrated code related to classic video type (e.g. MPEG) to Widgetfy.
Migrated Real Media file support to Widgetfy

At this point, all obsoleted embed rendering code are gone.