Fixes XSS injection

REF: https://gitlab.com/gitlab-org/gitlab-ce/issues/15434

**Without the fix**

![xss1](/uploads/0a7b0b15fb87066965a7c73f1dbaa815/xss1.gif)


**With the fix**

![xss2](/uploads/473cfa0aa80656f24c58aebf1fd97fff/xss2.gif)


See merge request !1952