Preference to open external links in new tabs

[PortableProgrammer]

Proposal

Add a user preference that makes external links (post URLs to non-reddit/non-lurker domains, oembed iframes, expanded ExtLinks) open in a new tab via target="_blank" rel="noopener noreferrer". Internal navigation (subreddit pages, comment threads, dashboard) stays in the same tab.

Definition of "external"

Any URL whose host is not the lurker host itself and not reddit.com / redd.it (those get rewritten to relative URLs by #34).

Schema

New column pref_openExternalInNewTab (boolean, default 0).

Touches

• DB migration in src/db.js
• prefToColumn map in src/routes/index.js:566-571
• get_user_prefs helper at src/routes/index.js:952
• Dashboard checkbox in src/views/dashboard.pug
• Conditional target attribute on external anchors in post.pug, comment.pug, index.pug — easiest via a pug mixin or utility helper

Acceptance

With the preference enabled, clicking an external post link opens a new tab; clicking a subreddit name or comment thread stays in the same tab.