support for avatars

@drsnyder @rogatty 
https://wikia-inc.atlassian.net/browse/PLATFORM-529

Account for image types other than "normal" images located in bucket/images. For now we support "avatars", "images", and "" (which is equivalent to "images"). The URL scheme for normal images does not change, but for additional types they need to be specified after the bucket (wikia) name. For instance ```/common/avatars/7/7c/1271044.png/revision/latest``` will get "avatars" image type from the common bucket. ```/nelsontest3240/f/fd/Kitten_2182000b.jpg/revision/latest``` and ```/nelsontest3240/images/f/fd/Kitten_2182000b.jpg/revision/latest``` are equal. 
Should the `:image-type` extraction be moved to a separate function?