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