Postgres start/stop demo

[wangpatrick57]

Summary: Basic demo letting you start and stop Postgres.

Demo:
(Yes, this is a demo of the demo). The demo uses pgrep postgres to show that no Postgres instance is running. Then the user is able to start/stop Postgres on the web UI. The results of that are verified using pgrep postgres.

demo_demo.mov

Details:

• Uses streamlit for the frontend.
• Uses the recently factored out PostgresConn in the backend.
• Right now is only deployable locally. In the future we'll want to deploy it on a CMU-DB machine so people can play around with it.
• It currently relies on the same workspace used in development. In the future, we may want to give the demo its own workspace. The setup process of this workspace would then require more scripting.