This takes part of the work being done in #96 that was reverted but still useful.
Note Tailwind and Fresh weren't upgraded because there's no security vulnerability in either, and I have found the new versions to be worse in performance. Thos will likely stay at those fixed versions going forward.
This implements optional SSO via OIDC for logging in and signing up (for the first admin sign up or if sign up is allowed). The most requested feature!
Tested with Authentik and Google!
It includes a new `SimpleCache` interface (in-memory, using [`caches`](https://developer.mozilla.org/en-US/docs/Web/API/Window/caches)) for storing the state and code challenges.
Closes#13
