ImgLzy: limit WebP thumbs to PNG and JPG only
ImgLzy: use WebP for PNG and JPG thumbs only

* create AM group
* code linting
ImgLzy: add a unit test
Applying feedback from @lizlux
run js-beautify
ImgLzy: fix syntax error in regex
Serve WebP thumbs for 1/16 of traffic
Merge branch 'dev' into img-lzy-limited-webp-support
Merge branch 'dev' into img-lzy-limited-webp-support

Conflicts:
	extensions/wikia/AssetsManager/config.php
Merge pull request #3257 from Wikia/img-lzy-limited-webp-support

Images lazy loading: limited webp support