Fix "size" URL parameter converion to be int value or "full" literal.

CacheManager expects value of "size" parameter in it's methods to be either
integer or str with value "full"(this was inferenced from
config['ALLOWED_SIZES']). The problem was that values that seems to be valid
(like str "200") weren't converted to integers and later failed the check
"size in config['ALLOWED_SIZES']". Fixed by using standard flask parameter
type conversion with extra route config for value "full".