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
If you're using a reverse proxy like Cloudflare Tunnels, you can now set `CONFIG_SKIP_COOKIE_DOMAIN_SECURITY="true"` to avoid login issues.
Also makes some UX tweaks to Expenses, and fixes a style issue for Chrome in Windows (#44).
Fixes#43Fixes#44
