Make sure `window.opener` is null when opening tabs for users' links

Thanks to Hamid Ashraf (/u/hamihax) for the report! A lot of
user-agents allow windows and tabs to navigate their openers through
`window.opener.location`.

This creates a potential phishing risk where a victim with the
"open links in a new window" pref clicks a link submitted by an
attacker. The attacker's page may then navigate the reddit page that
opened it. When the user closes the tab with the attacker's submitted
content, they'll be greeted with an attacker controlled page, and
they would be less likely to notice since the tab previously contained
a page served by reddit.

IMO this is really a flaw in same-origin policy, but the behaviour is
present in a number of browsers.