Improve Label form and reduce duplication

Currently we have two nearly-identical label forms:

• Admin: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/admin/labels/_form.html.haml
• Shared: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/shared/labels/_form.html.haml

We should remove the admin-specific version and standardize on the "shared" one.

The UX needs some attention as well:

• "Choose any color" ends in a period; "Or you can choose ..." doesn't
• "Or you can choose one of suggested colors below" needs grammatical fixes