COMMONS-355 | [CKEditor] Allow to customize Shift Enter mode

**Problem Analysis:**
* `shiftEnterMode` option is always set to the same value as the `enterMode`.

**Fix Description:** 
* Add a new instance variable in the `org.exoplatform.webui.form.UIFormRichtextInput` to hold the `shiftEnterMode` (default to `2` as a value if not specified as an options which means it inserts a linebreak `<br/>` in the CKEditor).