[MOBILE] Improve mobile experience (#812)

- Add `stackable` to `very relaxed` pages, such they don't take extra
margins from the space.
- Add margin bottom to the user item in the navbar, so it doesn't look
like crammed.
- Make the migrate page usable, by reducing three to two cards in a row
and making the icon smaller.

Co-authored-by: Gusted <postmaster@gusted.xyz>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/812

web_src/css/modules/card.css

@@ -132,3 +132,9 @@ a.ui.card:hover {
   margin-left: 1em;
   margin-right: 1em;
 }
+
+@media (max-width: 767px) {
+  .ui.two-mobile.cards > .card {
+      width: calc(50% - 2em);
+  }
+}