More markdown compatibility fixes

* Escape " as "
* Enable GFM newlines
* Output a linebreak after <br> in the generated markup

(imported from commit c007ec422054f9fc66a810b66aac70f70a2a1952)

static/js/echo.js

@@ -238,6 +238,7 @@ function escape(html, encode) {
     .replace(!encode ? /&(?!#?\w+;)/g : /&/g, '&')
     .replace(/</g, '&lt;')
     .replace(/>/g, '&gt;')
+    .replace(/"/g, '&quot;')
     .replace(/'/g, '&#39;');
 }
 
@@ -340,6 +341,11 @@ $(function () {
         return out;
     };
 
+    // Put a newline after a <br> in the generated HTML to match bugdown
+    r.br = function () {
+        return '<br>\n';
+    };
+
     // Disable ordered lists
     // We used GFM + tables, so replace the list start regex for that ruleset
     // We remove the |[\d+]\. that matches the numbering in a numbered list

static/third/marked/lib/marked.js

@@ -513,7 +513,7 @@ inline.zulip = merge({}, inline.breaks, {
   emoji: /^:([A-Za-z0-9_\-\+]+?):/,
   usermention: /^(@\*\*([^\*]+)?\*\*)/m,
   realm_filters: [],
-  text: replace(inline.text)
+  text: replace(inline.breaks.text)
     (']|', '@:]|')
     ()
 });