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

frontend/src/components/RemoteAuthRequest.vue

@@ -196,7 +196,7 @@ async function startRemoteAuth() {
       } else if (msg.status === 'authenticated') {
         // Success
         completed.value = true
-        emit('authenticated', { session_token: msg.session_token })
+        emit('authenticated', { exchange_code: msg.exchange_code })
         break
       } else if (msg.status === 'denied') {
         // Explicitly denied by the authenticating device