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

CacheManager expects value of `size` parameter in it's methods to be either
__`int`__ 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"_.