FEATURE: Embed topics list on remote sites via Javascript API. (#8008)

This adds support for a `<d-topics-list>` tag you can embed in your site
that will be rendered as a list of discourse topics. Any attributes on
the tag will be passed as filters. For example:

`<d-topics-list discourse-url="URL" category="1234">` will filter to category 1234.

To use this feature, enable the `embed topics list` site setting. Then
on the site you want to embed, include the following javascript:

`<script
src="http://URL/javascripts/embed-topics.js"></script>`

Where `URL` is your discourse forum's URL.

Then include the `<d-topics-list discourse-url="URL">` tag in your HTML document and it will
be replaced with the list of topics.

public/javascripts/embed-topics.js

@@ -0,0 +1,47 @@
+(function() {
+  function postMessageReceived(e) {
+    if (!e) {
+      return;
+    }
+
+    if (e.data && e.data.type === "discourse-resize" && e.data.embedId) {
+      var elem = document.getElementById(e.data.embedId);
+      if (elem) {
+        elem.height = e.data.height + "px";
+      }
+    }
+  }
+  window.addEventListener("message", postMessageReceived, false);
+
+  document.addEventListener("DOMContentLoaded", function(event) {
+    var lists = document.querySelectorAll("d-topics-list");
+
+    for (var i = 0; i < lists.length; i++) {
+      var list = lists[i];
+      var url = list.getAttribute("discourse-url");
+      if (!url || url.length === 0) {
+        console.error("Error, `data-discourse-url` was not found");
+        continue;
+      }
+      var frameId =
+        "de-" +
+        Math.random()
+          .toString(36)
+          .substr(2, 9);
+      var params = ["discourse_embed_id=" + frameId];
+      list.removeAttribute("discourse-url");
+
+      for (var j = 0; j < list.attributes.length; j++) {
+        var attr = list.attributes[j];
+        params.push(attr.name.replace("-", "_") + "=" + attr.value);
+      }
+
+      var iframe = document.createElement("iframe");
+      iframe.src = url + "/embed/topics?" + params.join("&");
+      iframe.id = frameId;
+      iframe.frameBorder = 0;
+      iframe.scrolling = "no";
+      list.appendChild(iframe);
+    }
+  });
+})();