Add Distributed Snowflake Server Support for WebExt

This merge request adds Relay URL support to the web version of the snowflake proxy.

It has been tested with Firefox on https://wispy-sky-e63c.shelikhoo.workers.dev/newbroker/embed.html.

Things to look at:

• It should be additionally tested on Chrome to make sure there is no regression.
• It does not make allowed prefix user-configurable.
• Any recommended changes to JavaScript coding style are welcomed.