Upcast deprecated table cellpadding attribute

The demo is split into three columns:

1. Editor
2. Source data (user agent styles)
3. Source data wrapped in the .ck-content class

By default, each table uses the deprecated cellpadding="20" attribute. The last two tables are exceptions: one sets cellpadding="0", and the other intentionally uses an cellpadding attribute. A descriptive caption is provided for each table.