Consider `superset:ui` as (blocking) version of `superset:up` and `superset:down`

@pnadolny13 - In the meltano project, we considered a generic ui command for any utility that has a web UI. As we discussed it, that would be a blocking command, which when killed would also kill the UI.

I see here in this package we have up and down as commands. What do you think about feasibility and practicality of having ui as a command - either in place of or in addition to the up and down commands?

Linked issues:

• Let `meltano ui` also start UIs for Airflow and... (meltano#2606 - closed)
• Add a `ui` command for the `Airflow` plugin (meltano#2944 - closed)