XWIKI-10407: Polish Flamingo menu

- removed leftovers from Junco skin. Flamingo is not having the .actionmenu, .submenu, etc. classes since the templates are using the standard Bootstrap navbar, dropdown-menu, etc.
XWIKI-10407: Polish Flamingo menu

- removed the border-radius from #menuview
XWIKI-10407: Polish Flamingo menu

- added missing colours for links and backgrounds