CE-1125 Merge ImageReview fixes from #release-252-image-review

This massive PR includes important fixes for Special:ImageReview:
* Improved conditions for sending an image to review
* Self-cleaning mechanism for image_review queue
* Counters reset mechanism
* Removal of the Invalid queue
* Use of Vignette for thumbnails
* Lots of refactoring and code separation
* Lots of documentation