DAR-2135 P2 fix after implementing lazy loaded RelatedPages

During implementation of lazy loaded RelatedPages module we missed that `FilePageController` calls one method directly.

To make it working again we:
* created new controller which is called in `FilePageController` (the new controller's `section()` method was a slightly changed copy of [previous `RelatedPages::executeIndex()` method](https://github.com/Wikia/app/blob/dc7db17c5dce4da0d934d8fccfc9c34127820fe5/extensions/wikia/RelatedPages/RelatedPagesController.class.php#L11)),
* changed `renderView()` params in `FilePageController` so it calls `section` method instead of `index` (so we re-used same for all skins mustache template).

Lazy loading this module on file page is more complex task due to the different `article id` than the `wgArticleId` on which related pages are pulled from database.

@hakubo @RafLeszczynski feel free to take a look, please :)