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/Integration.md

@@ -91,7 +91,7 @@ if (response.status === 401 || response.status === 403) {
 Get current user details:
 
 ```js
-const user = await apiJson('/auth/api/user-info', { method: 'POST' })
+const user = await apiJson('/auth/api/user-info', { method: 'GET' })
 // Returns: { uuid, display_name, credentials, sessions, permissions, ... }
 ```