* Factorise common code in ImagePage::delete() and allow normal page deletion if...

* Factorise common code in ImagePage::delete() and allow normal page deletion if $wgUploadMaintenance is true
* Moved $wgUploadMaintenance check after permissions and read only, so that the user doesn't think the error is temporary if he both doesn't have the permission and $wgUploadMaintenance is true
* Show normal error page when $wgUploadMaintenance and added a message for the error title
* Moved watchlist updating to FileDeletForm::execute(), it has nothing to do in doDelete() (would also be executed for api requests, etc.)
* Added $user parameter to FileDeletForm::doDelete() to pass the user doing the action
* Use WikiPage instead of Article