OAuth2 OpenID Connect provider support, API and DB refactoring (#3)

Allows Paskia to authenticate the user to a client site.
- User friendly client registration flow on the admin app
- Redirect-based authentication flow (per spec)
- Backchannel logout both ways to keep sessions synchronized
- Groups integrated with Paskia's permission system
- Adds email, preferred username and telephone fields on user profile
- All new user basic info layout to show the new information, better looks
- API and DB structures redesigned
- Various unrelated fixes to theming and layout

docs/API.md

@@ -15,7 +15,7 @@ For integrating Paskia with your app frontend, see [integration](Integration.md)
 | Method | Path | Used for | Notes |
 |---:|---|---|---|
 | GET | `/auth/api/settings` | Paskia configuration | Returns RP info + base paths + session cookie name |
-| POST | `/auth/api/user-info` | Full user profile | Basic information, credentials, sessions, permissions |
+| GET | `/auth/api/user-info` | Full user profile | Basic information, credentials, sessions, permissions |
 | POST | `/auth/api/logout` | Terminate session and delete session cookie | Signs out of the current site |
 | POST | `/auth/api/validate` | Validate and renew session cookie | Optional query: `perm=` (repeatable), `max_age=` |
 | GET | `/auth/api/forward` | Validate access (Caddy/Nginx) | 204 on success; 401/403 otherwise (HTML if requested) |