Bug 1835444 - Make `HTMLEditor` never move unremovable nodes r=m_kato, a=dmeehan

The `outdent` handler faces unexpected scenario due to the `<blockquote>`
element becomes not editable due to the `onstart` listener. The event listener
runs when the `<marquee>` moves from its container to new left node at splitting
it. Therefore, we need to put a lot of "removable node checks" before moving
nodes.

Differential Revision: https://phabricator.services.mozilla.com/D179421