Fully automate dev setup with Gitpod

[san-nat]

This commit implements a fully-automated development setup using Gitpod.io, an online IDE for GitHub and GitLab that enables Dev-Environments-As-Code. This makes it easy for anyone to get a ready-to-code workspace for any branch, issue or pull request almost instantly with a single click.

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[matyasf]

We might look at this in the future