UnknownContentHandler can be configued to handle models that
belong to extensions that have been undeployed:
$wgContentHandlers['xyzzy'] = 'UnknownContentHandler';
This way, no errors will be thrown when trying to access
pages with the unsupported model. Instead, an error message is
shown, and editing is prevented.
This patch also improves handling of non-editable content in
EditPage and in DifferenceEngine.
Bug: T220608
Change-Id: Ia94521b786c0a5225a674e4dc3cb6761a723d75b
6906a7728c