LDEV-4123: Removing the highContrast theme that is no longer used. Users with this theme should now revert to the default theme. The code that handled invalid themes was not correct and was stopped users logging in - now if the theme id in the database is null or if the theme id points to a non-existent theme then it can still log in using the default theme, and next time the user updates their profile the theme id will be fixed.