UI fixes related to flash/validation messages when saving user settings

[roland-ruedenauer]

Fixes two issues we currently have with saving user settings

• validation error message getting lost when saving user settings (caused by a client reload call made even if errors were present)
• avoid displaying the same flash messages twice (XHR case: the client code is fully responsible dealing with flash messages)

[UlrichB22]

I checked the Usersettings view and the User CSS URL in the tab Appearence. When entering "@wrong.css",
it now shows the applied value in red and the error message.

The next problem is the size and location of the message, which is not optimal.

[roland-ruedenauer]

Thanks for testing this.

The latest commit tries to improve upon what you've shown above. Please note that I didn't check interference with other themes. Further UI improvements should probably handled in a separate merge request.