converting from Redactor - `htmlSupport` predefined options

[i-just]

Description

When converting fields from Redactor to CKEditor, if you have the “html” button in your Redactor config, it’ll get “translated” into the “sourceEditing” button in the CKEditor field. Since Redactor allowed classes, attributes, and styles to be added via the HTML editing mode (provided HTML Purifier was configured to support them, too), CKEditor should also allow those.

Related issues

#263