UX: Trigger primary action in modals using Enter

A modal's primary action (blue button in the default theme) can now be invoked
by hitting Enter on the keyboard. This applies to all modals that aren't strict
forms as long as the focus is not on a textarea element.

test/javascripts/acceptance/modal-test.js.es6

@@ -52,3 +52,38 @@ QUnit.test("modal", async function(assert) {
     "ESC should not close the modal"
   );
 });
+
+acceptance("Modal Keyboard Events", { loggedIn: true });
+
+QUnit.test("modal-keyboard-events", async function(assert) {
+  await visit("/t/internationalization-localization/280");
+
+  await click(".toggle-admin-menu");
+  await click(".topic-admin-status-update button");
+  await keyEvent(".d-modal", "keydown", 13);
+
+  assert.ok(
+    find("#modal-alert:visible").length === 1,
+    "hitting Enter triggers modal action"
+  );
+  assert.ok(
+    find(".d-modal:visible").length === 1,
+    "hitting Enter does not dismiss modal due to alert error"
+  );
+
+  await keyEvent("#main-outlet", "keydown", 27);
+  assert.ok(
+    find(".d-modal:visible").length === 0,
+    "ESC should close the modal"
+  );
+
+  await click(".topic-body button.reply");
+
+  await click(".d-editor-button-bar .btn.link");
+
+  await keyEvent(".d-modal", "keydown", 13);
+  assert.ok(
+    find(".d-modal:visible").length === 0,
+    "modal should disappear on hitting Enter"
+  );
+});