Add a delete button to the edit label pages

Proposal

Add Delete label button to label edit page. This includes Admin, Project and Group labels.

Implementation plan

1. Refactor app/views/admin/labels/edit.html.haml#L7 to use app/views/shared/labels/_form.html.haml
2. Add Delete label button to app/views/shared/labels/_form.html.haml using app/assets/javascripts/delete_label_modal.js

Availability & Testing

• White-box tests already exist for various label actions, such as create, edit, etc. (ex: see gitlab/spec/features/groups/labels/create_spec.rb). Consider adding a white-box test for deleting a label from the edit page for groups, projects and admin as well.