WIP: OIDC-WebView implementation

Implement a WebView to connect to our Backends KeyCloak instance for Authentication.

• Which Route to use: auth.tinycampus or geronimo.tinycampus? (maybe see tc-server!97 (closed))
  • For now the second link on: 'https://auth.tinycampus.de/login'
  • Provide a shorthand for the login-site.
  • TinyCampus-Login is available under 'https://auth.tinycampus.de/authorization/tinycampus'
  • Provide a RegExp for NavigationDecisions based on known trustworthy URLs.
• Which Plugin for WebView?
  • webview_flutter seems promising
• How to receive Token from WebView?
  • JavaScript-Channel and -Messages?
  • In the same session send a GET-Request to https://auth.tinycampus.de/oauth/create_token after logging in. (this can be detected through the reirect to https://auth.tinycampus.de) THIS WORKS!
  • Android
  • iOS
• Styling
  • Layout and Button (waiting on !258 (merged))
  • CircularProgressIndicator while WebView is loading.

part of #172 (closed) closes #166 (closed) closes #114 (closed)

Known Issues:

• Logging out does not reset WebView/LoginService so that you drop right into last Account.
  • Might be cause by #2