DEV: Deprecate theme, add admin notice (#190)

The Horizon theme will soon be installed by default in Discourse, and
all development will be done there. This commit shows a warning for
admins telling them to switch to the new theme.

javascripts/discourse/api-initializers/horizon.gjs

@@ -1,8 +1,26 @@
+import { addGlobalNotice } from "discourse/components/global-notice";
 import { apiInitializer } from "discourse/lib/api";
+import getURL from "discourse/lib/get-url";
+import { currentThemeId } from "discourse/lib/theme-selector";
 import ExperimentalScreen from "../components/experimental-screen";
 import UserColorPaletteSelector from "../components/user-color-palette-selector";
 
 export default apiInitializer("1.8.0", (api) => {
+  if (api.getCurrentUser()?.admin) {
+    const themeId = currentThemeId();
+    const themeURL = getURL(`/admin/customize/themes/${themeId}`);
+    const horizonThemeUrl = getURL(`/admin/customize/themes/-2`);
+    addGlobalNotice(
+      `<b>Admin notice:</b> you are using the <em>Horizon (beta)</em> theme. This theme is now installed automatically in Discourse core. You should <a href="${themeURL}">uninstall this theme</a>, and reconfigure the <a href="${horizonThemeUrl}">preinstalled Horizon theme</a>.`,
+      "loading-slider-theme",
+      {
+        dismissable: true,
+        level: "warn",
+        dismissDuration: moment.duration("1", "hour"),
+      }
+    );
+  }
+
   api.renderInOutlet("above-main-container", ExperimentalScreen);
   api.renderInOutlet("sidebar-footer-actions", UserColorPaletteSelector);