FEATURE: Reimplement SiteSetting.max_oneboxes_per_post. (#6668)

Previously, the site setting was only effective on the client side of
things. Once the site setting was been reached, all oneboxes are not
rendered. This commit changes it such that the site setting is respected
both on the client and server side. The first N oneboxes are rendered and
once the limit has been reached, subsequent oneboxes will not be
rendered.

test/javascripts/acceptance/composer-onebox-test.js.es6

@@ -0,0 +1,47 @@
+import { acceptance } from "helpers/qunit-helpers";
+import { INLINE_ONEBOX_CSS_CLASS } from "pretty-text/inline-oneboxer";
+
+acceptance("Composer - Onebox", {
+  loggedIn: true,
+  settings: {
+    max_oneboxes_per_post: 2,
+    enable_markdown_linkify: true
+  }
+});
+
+QUnit.test(
+  "Preview update should respect max_oneboxes_per_post site setting",
+  async assert => {
+    await visit("/t/internationalization-localization/280");
+    await click("#topic-footer-buttons .btn.create");
+
+    await fillIn(
+      ".d-editor-input",
+      `
+http://www.example.com/has-title.html
+This is another test http://www.example.com/has-title.html
+
+http://www.example.com/no-title.html
+
+This is another test http://www.example.com/no-title.html
+This is another test http://www.example.com/has-title.html
+
+http://www.example.com/has-title.html
+      `
+    );
+
+    assert.equal(
+      find(".d-editor-preview:visible")
+        .html()
+        .trim(),
+      `
+<p><aside class=\"onebox\"><article class=\"onebox-body\"><h3><a href=\"http://www.example.com/article.html\">An interesting article</a></h3></article></aside><br>
+This is another test <a href=\"http://www.example.com/has-title.html\" class=\"${INLINE_ONEBOX_CSS_CLASS}\">This is a great title</a></p>
+<p><a href=\"http://www.example.com/no-title.html\" class=\"onebox\" target=\"_blank\">http://www.example.com/no-title.html</a></p>
+<p>This is another test <a href=\"http://www.example.com/no-title.html\" class=\"\">http://www.example.com/no-title.html</a><br>
+This is another test <a href=\"http://www.example.com/has-title.html\" class=\"${INLINE_ONEBOX_CSS_CLASS}\">This is a great title</a></p>
+<p><aside class=\"onebox\"><article class=\"onebox-body\"><h3><a href=\"http://www.example.com/article.html\">An interesting article</a></h3></article></aside></p>
+      `.trim()
+    );
+  }
+);