Make data loading a bit better in ApiDelete.

Make data loading a bit better in ApiDelete.

* Use WikiPage::newFromID() directly instead of Title::newFromID() (for now it does not change anything but it will soon)
* Directly load the data from the master when creating a page from its name, it avoids both having out-of-date information
  when doing the existence check and a second database query from LinkCache::addLinkObj() when calling (WikiPage|Title)::exists()
* Moved the page existence check to only be executed when creating a title from its name since if we have a WikiPage from its ID it must exist.

Change-Id: Ifaff725d955ce111c46d6b7f00610191820f3ced