Label link when filtering labels leads to 404 page

To reproduce:

• Go to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests?label_name%5B%5D=Frontend
• Click the "Frontend" label
• You'll be taken to https://gitlab.com/gitlab-org/gitlab-ce/labels/127616, which 404's

I don't think we have a show view for labels, so no page would exist for that route. Should we have one, or should we change/remove the link?

cc: @jschatz1