Previously we used both a `:namespace_id` and a `:repository_id` in the
repository routes, which originally made sense for project repositories,
but was confusing with personal snippets where `:namespace_id` was just
the `/snippets` prefix and not an actual namespace.

For group wiki repositories we also need to support toplevel groups,
where we only have a namespace path.

So we combine `:namespace_id` and `:repository_id` into a single
`:repository_path` route argument, and refactor the related controllers
and internal APIs accordingly.

The repository path is mainly used in `Gitlab::RepoPath` to determine
the container for the repository. In the `GitAccess` classes it's only
used in `GitAccessProject` when auto-creating project repositories,
where we need to know in which namespace to create the new project.