ikiwiki/ todo/ limit the markup formats available for editing

For aggregate to work, I have to have the html plugin enabled, and this allows users to create html pages via the standard edit form. It would be good if I could tell IkiWiki that I don't want certain page types to be editable (but still enabled to let e.g. aggregate/inline work. So by telling IkiWiki that e.g. html pages are uneditable (in the setup file), people would no longer

--madduck