css: Use classnames for settings page .sidebar li items.

Having the `*` selector there affects the performance. See
https://chat.zulip.org/#narrow/stream/6-frontend/topic/CSS.20selector.20performance/near/1845719
for more details.
We've used `sidebar` instead of `sidebar-wrapper` for the rules in
dark-theme also, since that is the rule being followed for light theme.
We've introduced a new class called `sidebar-item`.

web/styles/dark_theme.css

@@ -697,8 +697,8 @@
     .member-list-box,
     .member-list-box .member_list_container .member-list td,
     #subscription_overlay .settings-radio-input-parent,
-    #settings_page .sidebar-wrapper,
-    #settings_page .sidebar-wrapper *,
+    #settings_page .sidebar,
+    #settings_page .sidebar .sidebar-item,
     #recent_view_table table td {
         border-color: hsl(0deg 0% 0% / 20%);
     }