Implemented comment thread target picker with new dropdown components (#295)

* First draft of new relation picker and usage in comments
---------

Co-authored-by: Charles Bochet <charles@twenty.com>

front/src/modules/ui/components/menu/DropdownMenu.tsx

@@ -17,4 +17,6 @@ export const DropdownMenu = styled.div`
   height: fit-content;
 
   width: 200px;
+
+  z-index: ${(props) => props.theme.lastLayerZIndex};
 `;