Usability improvements for the Web Admin Suite

The following usability improvements for the Web Admin Suite have been made.

Flow for non-admin users trying to access the application improved (#664235)

When a user without administrator permissions has an existing OIDC session and tries to access the Web Admin Suite, an information page with a logout button is shown. It explains the lack of access permissions and allows the user to log out, so that he can log in again with an appropriate user. Alternatively, the user can log out using the Logout option in the avatar.