Consider changing the route for clojars.org/:key from showing users to showing groups

[danielcompton]

Currently there are two routes to see a user: /:username and /users/:username. This has a few impliciations.

• There are two locations for the same resource, and neither are canonical or redirect to the other.
• It means that URLs for groups aren't as nice as they could be. For example, if you have the URL https://clojars.org/bilus/pipes, going to https://clojars.org/bilus doesn't take you to the bilus group, but the bilus user.

It might be good to change the URL structure so /:identifer first tries for group, then artifact. Some thought needs to be put into how we handle artifacts with the same group and name. Thoughts on this are welcome. This would be a breaking change in how the URLs work, so I don't want to rush into this.

https://github.com/clojars/clojars-web/blob/master/src/clojars/routes/user.clj#L37-L38