Add a "Clean Paste Content Automatically" option for CKEditor

Here's what I propose:

• implement (2) and enable it by default
• document a way to disable it from the Advanced Configuration text area. This is only for (2), it doesn't make sense to disable (1).
• don't introduce a UI for it in the administration section because, as you can see from the attached image, there's a lot to explain about what it does and it will simply confuse the users, plus we don't want uses to easily disable it because we want them to focus on the content and leave the presentation to be handled by the XWiki skin.

Sounds good! This will make it harder for users to disable a feature they don't fully understand

If you want to disable the new behavior only for paste from Word use the following in the Advanced Configuration:

// See https://dev.ckeditor.com/ticket/13093
config.applyPasteFilterAfterPasteFromWord = false;

If you want to disable the new behavior for any source use:

// See https://docs.ckeditor.com/ckeditor4/docs/#!/api/CKEDITOR.config-cfg-pasteFilter
config.pasteFilter = CKEDITOR.env.webkit ? 'semantic-content' : null;

Looks good to me (at least for now), provided we document how to skip 2 if need be.

The option to "Clean Paste Content Automatically" from the old WYSIWYG editor had two goals:

1. Ensure that the pasted HTML content is valid
2. Ensure that the corresponding wiki syntax is as simple as possible (i.e. keep only the HTML elements and styles that have dedicated wiki syntax markup)

CKEditor does (1) by default so what's missing is (2). Here's what I propose:

• implement (2) and enable it by default
• document a way to disable it from the Advanced Configuration text area. This is only for (2), it doesn't make sense to disable (1).
• don't introduce a UI for it in the administration section because, as you can see from the attached image, there's a lot to explain about what it does and it will simply confuse the users, plus we don't want uses to easily disable it because we want them to focus on the content and leave the presentation to be handled by the XWiki skin.