More markdown compatibility fixes

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

(imported from commit c007ec422054f9fc66a810b66aac70f70a2a1952)

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)
     (']|', '@:]|')
     ()
 });