FIX: Restore support for normalized plugin routes under admin/ (#35598)

Following 37648149c4, it became impossible
to resolve some deprecations with plugin admin templates. This commit
fixes that problem, and also resolves the deprecation for the
chat-integration plugin.

frontend/discourse/app/resolver.js

@@ -387,6 +387,7 @@ export function buildResolver(baseName) {
         let adminParsedName = this.parseName(`template:${namespaced}`);
         const candidates = [
           [parsedName, "discourse/admin/templates/"],
+          [parsedName, "admin/templates/"],
           [adminParsedName, "admin/templates/"],
           [adminParsedName, "discourse/admin/templates/"],
           [adminParsedName, "admin/"],

frontend/discourse/tests/unit/ember/resolver-test.js

@@ -431,6 +431,13 @@ module("Unit | Ember | resolver", function (hooks) {
       }
     );
 
+    lookupTemplate(
+      assert,
+      "template:admin/plugin-template",
+      "discourse/plugins/my-plugin/discourse/templates/admin/plugin-template",
+      "looks up templates in plugin discourse namespace with correct name"
+    );
+
     withSilencedDeprecations(
       "discourse.deprecated-resolver-normalization",
       () => {