MDL-52724 editor_atto: Scrub atto html for invalid ol, ul, and li tags
Try to correct broken ul/ol/li tags, as they have an outsized impact
on course layout. Uses basic regex and loops to track open and closed
tags.
Also adds a deep clean option to the HTML cleaner, that runs less
frequent, more intensive cleanings. This is because normally _cleanHTML
gets called after each keystroke, which could cause problems with
large content on weak systems.
Behat changes are a fix for setting multiline strings in Atto, and
creating a multiline match step.
