Introduce `--page-space-bottom` at 64px (#30692)

Previously we would always leave 80px space before the page footer, but
this is problematic with small viewport heights on projects page for
example. I think it' ideal that we use `--page-spacing` which is already
in use for spacing on top of the page.

The `secondary-nav` margin is also adjusted as I see no value why this
shouldn't be the same value.

web_src/css/base.css

@@ -29,6 +29,7 @@
   --checkbox-size: 15px; /* height and width of checkbox and radio inputs */
   --page-spacing: 16px; /* space between page elements */
   --page-margin-x: 32px; /* minimum space on left and right side of page */
+  --page-space-bottom: 64px; /* space between last page element and footer */
 }
 
 @media (min-width: 768px) and (max-width: 1200px) {
@@ -479,7 +480,7 @@ img.ui.avatar,
 
 .full.height {
   flex-grow: 1;
-  padding-bottom: 80px;
+  padding-bottom: var(--page-space-bottom);
 }
 
 .status-page-error {