Add UnknownContentHandler.

Add UnknownContentHandler.

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