Add script to make embedded iframes autosize (#4853)

public/embed.js

@@ -0,0 +1,43 @@
+(function() {
+  'use strict';
+
+  var ready = function(loaded) {
+    if (['interactive', 'complete'].indexOf(document.readyState) !== -1) {
+      loaded();
+    } else {
+      document.addEventListener('DOMContentLoaded', loaded);
+    }
+  };
+
+  ready(function() {
+    var iframes = [];
+
+    window.addEventListener('message', function(e) {
+      var data = e.data || {};
+
+      if (data.type !== 'setHeight' || !iframes[data.id]) {
+        return;
+      }
+
+      iframes[data.id].height = data.height;
+    });
+
+    [].forEach.call(document.querySelectorAll('iframe.mastodon-embed'), function(iframe) {
+      iframe.scrolling      = 'no';
+      iframe.style.overflow = 'hidden';
+
+      iframes.push(iframe);
+
+      var id = iframes.length - 1;
+
+      iframe.onload = function() {
+        iframe.contentWindow.postMessage({
+          type: 'setHeight',
+          id: id,
+        }, '*');
+      };
+
+      iframe.onload();
+    });
+  });
+})();