Fixes #827

Description

This PR aims to fix an issue on mobile (#827) where pasted content from certain sources omits the images. Related comment: #617 (review), and a clipboard snippet of html that reproduces the issue on mobile here: #827 (comment).

Steps to test:

1. Select content from this site, including text above and below the image above "The Visual Editor.". Alternatively, if you are using an emulator with shared clipboard, it suffices to copy the html from the snippet referenced above.
2. Create an empty paragraph block.
3. Paste the contents.

Types of changes

When some content is pasted, a code path in figure-content-reducer.js expects parentElement property to be implemented when finding the containing <p> element (ancestor) surrounding an image element. Because parentElement is not implemented in the DOM for the mobile environment, <img> elements contained within <p> elements are not properly unwrapped, and are thus removed in a subsequent call to cleanNodeList.

It was noted in an earlier PR that we may encounter similar issues in other code paths as a result of our limited DOM implementation. These changes implement the parentElement property on the Node.prototype, as well as the previousElementSibling and nextElementSibling properties on the Node.prototype for all node implementing the NonDocumentTypeChildNode pure interface.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.