Change Refresh Stream button to fetch latest src

[shizuka]

Refresh Stream button is currently just a link targeted at if_stream's iframe, using the same URL written to the page by the embed switcher.

Change the Refresh and Popout buttons to use a new generated backend file (bypass cache, maybe piggyback brst-timers.js?) containing the current stream URL. Reloading page will get latest URL by nature of loading page, Refresh Stream button will pull the backend file and update things with that, and Popout should be fed the same URL rather than rely on if_stream's src.