Make ApiEditPage use Article::newFromWikiPage() and add user to context

Make ApiEditPage use Article::newFromWikiPage() and add user to context

- Changed the creation of the Article object in ApiEditPage::execute() to
  use Article::newFromWikiPage(), this allows to re-use the already-existing
  WikiPage object in that method instead of having to create a new one.
  Article::newFromTitle() was used before, but for some reason this got
  reverted back to "new Article" with the merge of the Wikidata branch.
- Removed the call to WikiPage::clear() added in I1d28164d (2c27926); no
  longer needed since the WikiPage object is now already up to date after
  the edit.
- Added WikiPage and User objects to the context passed to the created
  Article object; follow-up to I74394282 (078334f).

Change-Id: I53088a42ef7592aaba7449ef3f84f77f2e07b2f5