Add automation for joining the GH organisation

[ewels]

At the moment, the process of joining the nf-core organisation is a bit of a faff. You have to add a comment to some arbitrary ancient issue and hope that someone notices and adds you.

It's possible to add users to the organisation via the GitHub API, so we should be able to make a web page where people can do this themselves.

Note that to avoid people arbitrarily adding other people (deliberately, or through typos), we should probably require user authentication to do this. This adds some complexity, but I think it's probably worth it. As a bonus, having user auth would mean that we could programatically set their membership to public too - avoiding the need for #179