make wsproxy's proxying port pool configurable

[kyujin-cho]

webui's wsproxy currently determines proxy worker's listening port by choosing 100 random integers. We need to rewrite this logic, so that user can choose which ports to use when configuring webui.

• [ ] Choose how to feed option to wsproxy (e.g. environment variable, ...)
• [ ] Make wsproxy use chosen port pool instead of randomly selected ports if option is given

[lizable]

Updated Comments

Fwiw, this issue would be a good example of applying a multi-range slider. This issue is more likely to apply the environment variables right before executing wsproxy server. So providing a configurable user interface would be a further task.