add more URL validation

Turns out ShareX users and shell script authors are fucking retarded.

fhost.py

@@ -107,7 +107,7 @@ def shorten(url):
     if len(url) > app.config["MAX_URL_LENGTH"]:
         abort(414)
 
-    if not url_valid(url) or is_fhost_url(url):
+    if not url_valid(url) or is_fhost_url(url) or "\n" in url:
         abort(400)
 
     existing = URL.query.filter_by(url=url).first()