Add a deployment status using GitHub Actions

[platan]

2 years ago we created a bot that adds a comment the end of the PR with deployment status (#1708). The bot (https://github.com/apps/shields-deployment) was a https://github.com/bkeepers/workflow app running at ZEIT Now 1.0.

This year zeit.co announced Zeit Now 1.0 shutdown. On August 7, 2020, all Zeit Now 1.0 deployments were deleted.

Just before the shutdown, I moved Workflow instance to AWS Lambda. On the one hand it was a good move, because:

• we have few requests to handle and serverless is great in this case
• it's "free" in our case - `The AWS Lambda free usage tier includes 1M free requests per month (https://aws.amazon.com/lambda/pricing/)
• https://github.com/probot/serverless-lambda allowed to easily move to AWS Lambda (Workflow uses Probot)

On the other hand, I have some problems with the Workflow/serverless-lambda, because they use incompatible versions of Probot and it required some hacks to make them work properly (the result is stored in my private repo).

In this PR I remove Probot's configuration and migrate the whole solution to GitHub Actions.

platan/shields-deployment-status-example#55 is an example PR using similar GitHub Action

[shields-ci]

	Messages
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[chris48s]

nice. Simplifying this seems like a good move 👍

[calebcartwright]

Indeed, well done! I like having it defined and being able to manage entirely within repo too