2sxc 08.03.06 with fixes

Various fixes which work around DNN problems, mainly

1. JS-caching, which made the system feel broken after upgrades
2. iframing dnn-components, which showed the entire layout when you just wanted the wysiwyg or image-picker
3. installation issue, where dnn always started the code-installation at version 1, so it took very long
4. minor ui-improvements

https://github.com/2sic/2sxc/issues?q=is%3Aissue+milestone%3A%222sxc+08.03.06+Hotfixes%22+is%3Aclosed