Add Generic pre-redirect route for potentially unsafe URLs

Proposal

In !132892 (comment 1600812762), @pslaughter said:

WDYT of introducing a feature flag here, wrapping the UI portions in the FF check, so that we can move forward with this MR? Then we could collaborate on a follow-up to create this pre-redirect view?

This issue is to add this generic pre-redirect route so that Add support for Mastodon as user social (!132892 - merged) can be removed from behind the FF.

Blocks #428442 (closed)