Disable random slug generation for containers

@deiu posed:

During a POST request (with a Slug header set), the server may decide to create a random string that gets appended to the Slug value, if a resource with the same name already exists. While this feature is really useful for creating random resources (i.e. files) inside a container, it seems to be very annoying when applied to creation of containers.

I personally feel that most applications and users care a lot about the names of containers, and less so for resource names (which can sometimes be random).

My proposal is to drop this behavior when it involves container creation, and respond with a 409 - Conflict if a container with the same name already exists.

There has been some discussion on https://github.com/solid/solid/issues/42 with @melvincarvalho, @dmitrizagidulin, @sandhawke, and @bblfish and would like to invite more here.