Add API endpoint to retrieve the reverse dependencies.

This /api/v1/crates/foo/reverse_dependencies gives the full dependency
information about each crate that depends on foo (i.e. crates with
`[dependencies] foo = "..."`).
Add the /crate/foo/reverse_dependencies route.

This connects the web-app to the back-end API and allows it to display a
list of all the reverse dependencies of a crate as a
/reverse_dependencies subroute.

This uses the same format as the /versions route.