XWIKI-11408: Menu Application: Improve default look to make it better-looking with the Flamingo skin

- Because of the bug https://github.com/less/less.js/issues/1968 and after the commit https://github.com/xwiki/xwiki-platform/commit/e9b2bfc9b0b459edfbe79e3abcbfab8367368c2f , we cannot use :extend anymore
- Since I couldn't transform all the extends into mixins (failed for examples like '.dropdown-menu>li>a:hover', etc.) I copied the styling properties from Bootstrap
- In the future, if we fix the extend bug, we should clean this code (either of the properties or the extend comments)