feat(19.6.1-01): Roles screen

- Settings -> System -> Roles (summercms.cabana.roles, backend.manage_roles,
  order 110): name, code slugged from the name, description, checkbox
  permission editor with the editor's ceiling
- system roles (is_system or code-declared grants) keep their code and
  cannot be deleted; a role any admin holds (deleted ones included) cannot
  be deleted, the refusal names the role and the count
- role codes unique among roles

modules/cabana/backendscreens/controllers/roles/config_form.yaml

@@ -0,0 +1,19 @@
+name: backend::lang.user.role.name
+form: ~/plugins/summercms/cabana/models/userrole/fields.yaml
+modelClass: Backend\Models\UserRole
+defaultRedirect: summercms/cabana/roles
+
+create:
+    redirect: summercms/cabana/roles/update/:id
+    redirectClose: summercms/cabana/roles
+
+update:
+    redirect: summercms/cabana/roles/update/:id
+    redirectClose: summercms/cabana/roles
+
+messages:
+    create: backend::lang.user.role.new
+    update: backend::lang.user.role.edit
+    saved: backend::lang.messages.form.saved
+    deleteConfirm: backend::lang.user.role.delete_confirm
+    deleted: backend::lang.messages.form.deleted

modules/cabana/backendscreens/controllers/roles/config_list.yaml

@@ -0,0 +1,19 @@
+list: ~/plugins/summercms/cabana/models/userrole/columns.yaml
+modelClass: Backend\Models\UserRole
+title: backend::lang.user.role.list_title
+recordUrl: summercms/cabana/roles/update/:id
+noRecordsMessage: backend::lang.user.role.list_empty
+recordsPerPage: 20
+showCheckboxes: true
+showSetup: true
+
+toolbar:
+    buttons: [create, delete]
+    search:
+        prompt: backend::lang.list.search_prompt
+
+messages:
+    create: backend::lang.user.role.new
+    empty: backend::lang.user.role.list_empty
+    deleteConfirm: backend::lang.user.role.delete_selected_confirm
+    deleted: backend::lang.messages.list.deleted