MDL-41098 Text editor: Add HTML5 text editor to core.

This is an alternative to the TinyMCE text editor that is based on HTML5
content editable regions.

It has been designed to "gel" with Moodle and uses YUI for javascript,
Moodle standard dialogs and standard moodle buttons. It has also been designed
to work well on mobile devices and can collapse down to a narrow column if required.

Imported from the plugins database here:
https://moodle.org/plugins/view.php?plugin=editor_contenteditable

Original git repository here:
https://github.com/damyon/contenteditable
MDL-41098 contenteditable: Cleanups

Changes:
* Remove license and readme
* Remove unused and commented code
* Add missing phpdocs
MDL-41098 contenteditable: Make enabled by default

The order is tinymce -> contenteditable -> textarea
MDL-41098 contenteditable: Allow making images links.

The code was preventing you from creating a link with empty text - but
the test was converting a range to a string which did not include any
embedded html child nodes. So if you selected only an image, it would
look like empty text and prevent you from creating the link.
MDL-41098 contenteditable: link plugin lets you edit existing links

Before it would create a mess of nested links, now if you open the link plugin
when the cursor is within a link it will edit the existing link.
MDL-41098 contenteditable: Added doc comments + licenses to all.