Vote via GitHub comment

[marshall7m]

Allow the appropriate voters to approve/reject Terraform deployments via PR comments. Currently the only method to vote is via a comment until webhooks for reactions are supported (see discussion here). This should be the primary way voters can cast their vote since they could look at the PR changes and vote all within GitHub. This could be implemented by provisioning a separate GitHub webhook for PR comments that would kick off a Lambda Function that will:

1. Check if the GitHub user is a valid voter for that AWS account (maybe send a GitHub comment saying they are an invalid voter if not)
2. Update the vote count and send a Step Function success task token if the count is met
3. Send a PR comment using a GitHub bot account that notifies the user that their vote has been submitted