feat: Organisations admin screen with the avatar and the members manager

- controller golem15.user.organisations behind golem15.users.access_users
- list and form (name, slug preset from the name, description, avatar)
- members relation manager: link sets and unlink clears users.organisation_id
- deleting an organisation releases its members first
- Organisation gains Fillable, Rules, MorphName, AttachRelations, BeforeValidate
  and the Members relation; models.Slugify
- navigation side item organisations
- TestAdminOrganisations, TestAdminOrganisationMembers, TestSlugify

admin_navigation.go

@@ -32,6 +32,13 @@ func (*Plugin) Navigation() []pact.NavigationItem {
 					Permissions: []string{controllers.PermissionAccessGroups},
 					Controller:  "golem15.user.usergroups",
 				},
+				{
+					Code:        "organisations",
+					Label:       "golem15.user::lang.organisation.menu_label",
+					Icon:        "users-round",
+					Permissions: []string{controllers.PermissionAccessUsers},
+					Controller:  "golem15.user.organisations",
+				},
 			},
 		},
 	}