Some updates to EditPage and ExternalEdit:

Some updates to EditPage and ExternalEdit:
* Made ExternalEdit use a context
* Updated DifferenceEngine to use ExternalEdit to send the diff stuff to reduce code duplication
* Introduced ExternalEdit::useExternalEngine() to check whether to use the external edit or diff (except for the action, section and oldid which are still checked in Wiki.php) to remove code duplication; external diff can now also be controlled with externaledit or internaledit URL parameters
* Use $wgContLang to get the name of the "Special" namespace instead of user's language
* Modified the line breaks in the comment on the top of the control file so that the URL is on its own line
* Updated extension to call EditPage::edit() instead of EditPage::submit(), the latter will just call the former
* Updated extension to let core handle itself the ExternalEdit mode instead of doing this themself