Add class to page content to unify top margin (#13766)

* Add class to page content to unify top margin

Previously pages would individually set this margin but some didn't so
content would stick to the header without any space. Resolve this by
adding a new class that is added on all pages. The only place where we
remove this margin again is on the pages with menu or wrapper in the
header.

* fix admin notices

* fix team pages

* fix loading segment on gitgraph for arc-green

* fix last missing case

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/less/_explore.less

@@ -1,7 +1,6 @@
 .explore {
   .navbar {
     justify-content: center;
-    padding-top: 15px !important;
     margin-bottom: 15px !important;
     background-color: var(--color-navbar) !important;
     border-width: 1px !important;