Complete support for footnotes in the Content Editor

Problem to solve

In !71067 (merged) we introduced the ability to render Markdown footnotes in the Content Editor. However, because of a limitation in the backend Markdown renderer, we are unable to preserve the footnote ID.

Once #343365 (closed) is resolved, however, we should be able to complete support for footnotes in the Content Editor.

Proposal

After !61792 (merged) is merged, revisit the rendering of the footnotes to include the missing metadata and preserve the users' preferences.