Allow sorting reverse dependencies

[matthew-piziak]

/search accepts query arguments sort=downloads and sort=relevance.

/crates/{crate}/reverse_dependencies does not accept a sort query parameter. All reverse dependencies are in alphabetical order

"What are the most popular crates that use this dependency" is a valuable use case. For example, today I wanted to see good living examples of the iron crate, but was unable to distinguish them from other crates through the crates.io interface.

[carols10cents]

Sooo reverse dependencies are now ordered by downloads by default, but there still isn't a sort option, so now you can't sort alphabetically, so this bug still stands in a way :)

[matthew-piziak]

Thanks for doing this! :)