overview: Make the spacing dynamic depending on screen height

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>