overview: Make the spacing dynamic depending on screen height

The HTML/CSS sizing prototype that was done back in the day for GNOME 40 has
a dynamic spacing depending on the height of the monitor. Let's try to
follow that more closely and make the spacing dependent on screen height.

For the sizing prototype, see
https://gitlab.gnome.org/Teams/Design/os-mockups/-/tree/master/overview/sizing-prototype

This also allows removing of a bit of manually added spacing at the top of
the dash.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3278>

data/theme/gnome-shell-sass/widgets/_dash.scss

@@ -11,8 +11,6 @@ $dash_spacing: $base_margin * 0.5;
 
 // container for the dash
 #dash {
-  margin-top: $dash_edge_offset;
-
   // background behind item container
   .dash-background {
     background-color: $dash_background_color;

data/theme/gnome-shell-sass/widgets/_overview.scss

@@ -1,6 +1,6 @@
 /* OVERVIEW */
 
-.controls-manager, .secondary-monitor-workspaces {
+.secondary-monitor-workspaces {
   spacing: $base_padding * 2;
 }