Add support for blocking users via an environment variable

[karashiiro]

This allows users to be blocked through a new BLOCKED_USERS environment variable, which takes a comma-separated list of quoted user IDs. In the submission function, this list is just parsed with

JSON.parse(`[${process.env.BLOCKED_USERS || ""}]`)

I could've made it require the square brackets as well, but opted against it for user-friendliness. All of the code in this PR is adapted from a separate fork which is itself adapted from similar upstream code, I just cleaned all of that up a bit for this PR.

As mentioned in #15, this could be expanded upon with a managed DB like DynamoDB or MongoDB Atlas so that blocks can be added through bot commands - this is just a quick-and-easy way to accomplish the same thing in a slightly more limited form.

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[karashiiro]

Updated, how's this?