add a netlify function for a slack invite

[acao]

TODO:

• add yarn step to deploy config - perhaps we should move deploy config to netlify.toml? I wanted to avoid putting the package.json in the root, so that it doesn't give the impression that one needs to perform a yarn install to build the site.
• provide SLACK_TOKEN in netlify env variables, and replace the URL I think? or it will infer where to invite the user based on the token perhaps?

Documentation:

From this article on the netlify blog

For example, if you have a Netlify Form on your site, you can trigger a function automatically when receiving a new submission, simply by naming your function file submission-created.js. That's all!

[netlify]

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