add /verifydb command

[b9r5]

Description

This change adds a /verifydb command that:

• reads the DATABASE_URL environment variable
• attempts to establish a database connection and get a cursor
• executes select random()
• sends a response with the random value

Checklist

Before submitting this PR, ensure the following steps have been completed:

• Run the smoke test on your own server.
  • Run the cluster bot on your server:

    python discord-bot.py

  • Start a training run by with the slash command /run.
    You may need to exercise some judgement about the script and GPU type.
  • Wait for the training run to complete.
  • Copy the URL for the thread started by the cluster bot in response to
    your /run message ("Cluster Bot started a thread: ..."):
    • Click on the 3 dots (...) to the cluster bot's message.
    • Select Copy Message Link.
  • Using the copied URL, run the smoke test:

    python discord-bot-smoke-test.py copied_url

  • Verify that the smoke test script responds with:

    All tests passed!
    

  For more information on running a cluster bot on your own server, see
  README.md.

[b9r5]

We probably won't need the POSTGRES_* constants, but I wanted to leave them for now in order to verify we can connect to the database on Heroku. Once we verify that, we should be able to remove these constants.

[msaroufim]

cool! feel free to merge when ready