add manual front-end redirect

Nobody has claimed this yet.

It's a feature I wanted to avoid for a while but it has too many useful applications I guess:
add some API endpoint that propagates some redirect command to the frontend, e.g. redirect all users of `twitch/admin` to `angelthump/test`. Could build some random delay into it so it doesn't hit 1k users at once.

Applications:
- Streamer hosting feature: people could use this endpoint to make their viewers go to some other stream. Backend would have to check (if user not admin), if the source of the redirection is the `stream_path` of the initiator. That should suffice?
Reminder to include a black-list of streams that can never be a host target.

- If frontend gets updated we want to be able to make people reload cache-less. SInce there are many afk people on the site for days even, this would be nice. Redirect could be without destination to just reload.

- General moderation of content... E.g. move people away from some offending stream. (would be nice to be able to redirect to frontpage in this case, vs. some specific stream)

Start by locating the API entry points and the frontend code that handles navigation or reloads. Define the authorization, source and target rules, blacklist behavior, optional destination, and delayed propagation before implementation; done means each listed redirect use case is specified and covered by tests.

Written by the indexing model from the issue text.