OpenID Connect authentication support

[metasoarous]

Problem:
Identity management right now requires using the XID system. While extremely flexible, using this system requires setting up a separate web property which then embeds conversations.

Suggested solution:
OpenID Connect is a well supported standard among identity providers and would provide something closer to a solution for authenticating against a wide range of identity providers.

Alternative suggestions:

• Continue to just depend on the flexible XID system for identity management, and possibly put together some templates to make that process easier.

Additional context:

• This would only really benefit secondary deployments, since we're unlikely to support anything other than Twitter & Facebook on the main pol.is deployment (and may even remove these mechanisms at some point); Setting up and maintaining deployment is already quite a bit of work, so on the one hand, it's perhaps not a ton extra to create a site for embedding conversations, and adding the OpenID integration there, and many custom deployments will want conversations embedded in their own web properties anyway. On the other hand, many applications may not otherwise need conversations to be embedded, and this ultimately adds friction to an already involved process.
• This may ultimately be quite a bit of work on some of the more involved pieces of the voluminous server portion of the codebase (all the way through to the database), with important security implications at stake.
• See Additional login providers #97 for past discussion.

Status:
Given the flexibility of the XID system as it stands, and the complications mentioned above, CompDem is unlikely to fund this work ourselves. However, if there is interest in working on this, we'd consider reviewing and merging.

[rambip]

I want to deploy polis to my university. We have a SSO setup, so I would be very interested to have that feature.

I can work on it, I'm quite new to implementing authentication but I can learn how to do it.

I opened this discussion: #1779 to discuss how the login works right now.
The server.ts is a huge file, and maybe it would be nice to broke it up into smaller files. Right now it's hard to work on it.