The repository routes for project repositories are ambiguous and also
match project snippet repositories, so LFS requests for project snippets
will work but snippets are not ready yet to properly support LFS.

We can work around this by checking `#lfs_enabled?` on the `container`
instead of the `project`, which for snippets will be the snippet itself,
and `Snippet#lfs_enabled?` is currently hard-coded to return `false`.

To simplify things, we also remove the project-specific access check and
use `lfs_download_access?` instead to determine wether to expose the
existence of the project (404 response) or not (403 response), when
sending an error response. When LFS is disabled on the container we now
also send a 404 instead of a 403.