Add option to override mqtt param for wasm

[martinbosma]

Now you can do something like http://venus.local/gui-v2?mqtt=ws://123.123.1.2/websocket-mqtt

[blammit]

@chriadam could you check this one please?

[chriadam]

LGTM, but we should get Jeroen's PR in first, and then make this change directly to the index.html rather than modifying the patch file.