feat(cabana): settings entries that link to an admin controller

pact.SettingsItem gains an additive Controller field, the equivalent of a
WinterCMS registerSettings 'url' => Backend::url(...) entry. A link entry
declares no Model, Form or NewModel and needs no AdminFS; start-up fails
when it combines Controller with a singleton form or a model, or names an
unregistered controller. Settings codes stay one namespace.

GET /settings lists a link entry with its controller only when the
principal passes the item's permissions and may open the controller.
Registry.Setting never returns a link entry, so the singleton settings
endpoints answer 404 for its code. SettingsEntry carries controller,
empty for singletons; the OpenAPI document, generated SPA types and
settings fixture follow, and the pact and cabana READMEs and the
settings docs describe the link.

modules/cabana/example_settings_test.go

@@ -0,0 +1,24 @@
+package cabana_test
+
+import "git.golem15.com/golem15/summercms/modules/pact"
+
+// LinkedBlogPlugin adds a settings entry that opens the posts controller from
+// the Settings page.
+type LinkedBlogPlugin struct{ BlogPlugin }
+
+// Settings keeps the singleton page and adds a controller link.
+func (p *LinkedBlogPlugin) Settings() []pact.SettingsItem {
+	// docs:start settings-link
+	link := pact.SettingsItem{
+		Code:        "posts",
+		Label:       "acme.blog::lang.posts.title",
+		Description: "acme.blog::lang.posts.description",
+		Category:    "acme.blog::lang.plugin.name",
+		Icon:        "icon-pencil",
+		Order:       500,
+		Permissions: []string{"acme.blog.access_settings"},
+		Controller:  "acme.blog.posts",
+	}
+	// docs:end settings-link
+	return append(p.BlogPlugin.Settings(), link)
+}