Implement Label Show Page for individual Project and Group level labels

We discussed this page during a UGC session at the 2017 GitLab Summit in Greece

It's currently not possible to navigate to a page that represents an individual label. It's only possible to navigate to the label's edit page.

Let's implement the label show page so that a user can navigate to a page which displays the label's name and description.

What is it?

A page to view an individual labels properties

Why should someone use this feature?

To find more information about the label

What is the underlying (business) problem?

Currently, it's only possible to find a label's description through tooltips or finding a label in the index

How do you use this feature?

Navigate to the label's show page through the label index or a direct link

Make sure these are completed before closing the issue, with a link to the relevant commit.