Fix note--collapsed add class bug + erroneous paragraph insertion

This PR fixes issues: 

https://github.com/guardian/flexible-content/issues/1397
https://github.com/guardian/flexible-content/issues/1396

Tests have been written to cover the bugs.

Ive got a failing unit test which has been commented.  I'll resolve this asap but it doesn't affect or cause issues relating to these fixes.
Be good to have a comment about why this is necessary.