[FEATURE REQUEST] Make WebUI customizable

[kamatama41]

We are using asynq for some apps and each app has some environments like development, staging, production.
So there are a lot of asynq server in our platform and we monitor and operate each asynq server by asynqmon.

In order to avoid unintentional operation errors, we would like asynqmon to have an option to differentiate each asynqmon UI, for example like follows.