Add Github implementation

[SKernchen]

Right now it only works on HZDR gitlab.
Allow more options and also add Github

[poikilotherm]

As part of #3, @nheeb tried using Github OAuth 2 in this client-side only application.

We ran into a known, long standing limitation of Github's OAuth API:

• https://stackoverflow.com/questions/42150075/cors-issue-on-github-oauth
• OAuth web flow endpoints don't support CORS isaacs/github#330

Even our scholarly friends at the Alan Turing Institute had to put up with this in scivision#277. They have been using a deployment of https://github.com/prose/gatekeeper to work around this limitation.

Although Github has announced PKCE support in July, they still do not allow public OAuth clients (SPAs) to use it for authentication. They still do not distinguish between public and confidential clients, so they still have the CORS rule in place to avoid folks to spill the secret.

We need to consider our options here. Maybe we can host this very small gatekeeper (which is a confidential client) someplace with HIFIS? This would be beneficial for hermes init as well, as we'd be able to move on from the "device-flow only" approach it has right now.