Add --page-spacing variable, fix admin dashboard notice (#30302)

Fixes https://github.com/go-gitea/gitea/issues/30293 and introduce the
`--page-spacing` variable which holds the spacing between the elements
on the page. This is working vertically for all pages, including ones
that have fomantic grid, and horizontally for all that use
`flex-container`.

The `.page-content > :first-child:not(.secondary-nav)` selector uses
margin which in some cases enables to adjacent margins to overlap, which
is nice.

<img width="1320" alt="Screenshot 2024-04-06 at 01 35 19"
src="https://github.com/go-gitea/gitea/assets/115237/3e81e707-e9ff-4b7f-a211-3d98f4f85353">
---
<img width="1327" alt="Screenshot 2024-04-06 at 01 35 45"
src="https://github.com/go-gitea/gitea/assets/115237/aad196c0-9e21-4c06-ae59-7e33a76c61e1">
---
<img width="1321" alt="Screenshot 2024-04-06 at 01 35 31"
src="https://github.com/go-gitea/gitea/assets/115237/785f6c5d-08b6-4e66-aa16-aeca7cfed3ad">

web_src/css/modules/flexcontainer.css

@@ -2,7 +2,8 @@
 
 .flex-container {
   display: flex !important;
-  gap: 16px;
+  gap: var(--page-spacing);
+  margin-top: var(--page-spacing);
 }
 
 .flex-container-nav {