Solving this: https://linear.app/comm/issue/ENG-2045/add-highlight-effect-to-arrow-background-when-finger-is-on-thread-row
Reorganized `SidebarListModal`, `ChatThreadListSidebar` and `SidebarItem`, so that
`SidebarItem` doesn't hold button anymore, but rather is wrapped inside it.
Changed arrows SVG, so instead of short and long ones we have arrows extending in both ways and
short arrows that don't extend downwards, so they can finish list of thread sidebars.