Feature summary (what you would like to be able to do and where):
WikiEditor should be loaded on Special:Undelete revision pages.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
T418811 enabled CodeMirror on Special:Undelete, which loads if it is enabled. If it was toggled off in a WikiEditor window before though, there is no way to turn it on again in the Special:Undelete wikitext window.

Benefits (why should this be implemented?):
Better UX, same behaviour already exists when attempting to edit a protected page (check https://foundation.wikimedia.org/wiki/?action=edit or similar)