Pull Request for Issue #13665 .

Summary of Changes

This PR adds support for readonly/disabled editors

Testing Instructions

• Create a new custom field of the type "editor" and either set the permission so it is not editable (that's the case by default in the contact form) or enabled readonly or disabled in the field options
• Check the item where the field appears
• Test with all three core editors (TinyMCE, Codemirror, None)

Another test:

• Edit the editor formfield spec in a form XML and add a readonly="true" attribute to it. Eg edit /administrator/models/forms/article.xml and change

<field 
	name="articletext"
	type="editor"
	label="COM_CONTENT_FIELD_ARTICLETEXT_LABEL"
	description="COM_CONTENT_FIELD_ARTICLETEXT_DESC"
	filter="JComponentHelper::filterText"
	buttons="true"
/>

to

<field
	name="articletext"
	type="editor"
	label="COM_CONTENT_FIELD_ARTICLETEXT_LABEL"
	description="COM_CONTENT_FIELD_ARTICLETEXT_DESC"
	filter="JComponentHelper::filterText"
	buttons="true"
	readonly="true"
/>

• Check the item

Expected result

Editor should be readonly/disabled.

Actual result

You can edit the content fine.

Documentation Changes Required

https://docs.joomla.org/Editor_form_field_type can now have added attributes "readonly" and "disabled" (they both do the same thing here).