DEV: introduces {{concat-class}} helper (#17526)

Usage:

```
<button class={{concat-class "foo" this.bar (if true "baz")}} />
```

app/assets/javascripts/discourse/app/helpers/concat-class.js

@@ -0,0 +1,8 @@
+import { helper } from "@ember/component/helper";
+
+function concatClass(args) {
+  const classes = args.compact().join(" ");
+  return classes.length ? classes : undefined;
+}
+
+export default helper(concatClass);

app/assets/javascripts/discourse/tests/integration/helpers/concat-class-test.js

@@ -0,0 +1,44 @@
+import { assert, module, test } from "qunit";
+import { setupRenderingTest } from "discourse/tests/helpers/component-test";
+import { render } from "@ember/test-helpers";
+import { hbs } from "ember-cli-htmlbars";
+import { query } from "discourse/tests/helpers/qunit-helpers";
+
+module("Integration | Helper | concat-class", function (hooks) {
+  setupRenderingTest(hooks);
+
+  test("One class given", async function () {
+    await render(hbs`<button class={{concat-class "foo"}} />`);
+
+    assert.equal(query("button").className, "foo");
+  });
+
+  test("Multiple class given", async function () {
+    this.set("bar", "bar");
+    await render(hbs`<button class={{concat-class "foo" this.bar}} />`);
+
+    assert.equal(query("button").className, "foo bar");
+  });
+
+  test("One undefined class given", async function () {
+    this.set("bar", null);
+    await render(hbs`<button class={{concat-class "foo" this.bar}} />`);
+
+    assert.equal(query("button").className, "foo");
+  });
+
+  test("Only undefined class given", async function () {
+    this.set("bar", null);
+    await render(hbs`<button class={{concat-class null this.bar}} />`);
+
+    assert.notOk(query("button").hasAttribute("class"));
+  });
+
+  test("Helpers used", async function () {
+    await render(
+      hbs`<button class={{concat-class (if true "foo") (if true "bar")}} />`
+    );
+
+    assert.equal(query("button").className, "foo bar");
+  });
+});