Checkmk Werk 11246: Drop classic theme

ID: 11246 Title: Drop classic theme Component: Multisite Level: 2 Class: New feature Version: 1.7.0i1 With this change we remove the Classic theme. Even though there are still many fans of this theme who like to use our Classic Theme, we can no longer support this theme in the future due to cost and effort. Users of the Classic Theme will automatically be switched to the current default theme. The main reason for this change is that we will make massive changes to the user interface of Checkmk with the upcoming version. We can easily implement these changes in the modern and dark theme together. In addition, we would have had to redesign and implement some of these in the Classic theme, which we cannot afford.