WebHost: use redirect for /room form submission

[Berserker66]

What is this fixing or adding?

removes

if you enter a command into the bar in /room, then go back/forth via browser controls.

How was this tested?

comparing localhost to archipelago.gg

If this makes graphical changes, please attach screenshots.

Turns

into:

[black-sliver]

I personally prefer separate @app.get and @app.post to make it easier to read, but in this case it's not too bad.