👷 Add GitHub Actions workflow for automated invitations

[AnandChowdhary]

I've added a system that will automatically add people to the @eu organization.

How it works:

1. Someone creates a new issue with a title similar to "Join"
2. They automatically receive an invitation to join the organization
3. A comment is added on the issue telling the user that they should accept the invitation
4. The issue is automatically closed

It also has some check in place:

• If you've already received an invitation, you are skipped
• If the title is not "join" or "Join", the issue is skipped (could be a general discussion)

What to do before merging:

1. Add secret GT_PAT with permissions to manage the organization.
2. Ideally, I would like to be a maintainer of this project. This way, I can add my bot. Would it be possible to add me? For this, you can change my role from "Member" to "Owner".

This PR fixes #30.

[AnandChowdhary]

In the future, I as a maintainer I would also like to:

• Add a system to remind people to change their organization visibility "Public" for the badge to show on their profiles (when you're logged in, you can see the badge even if it's private, but people don't know that other users can't see it, currently 33% of all members are not public)
• Add repository health files in .github
• Add repository topics and details
• Update guidelines on README.md
• Update https://eu.github.io and host the README there

[AnandChowdhary]

@3x did you have a chance to look at this PR? Thanks!