feat(ui): redesign migration selection screen (#6795)

Ref https://codeberg.org/forgejo/design/issues/9.

Changes:
* Updated SVGs which had inconsistent paddings making them look off
* Better usability on mobile
* Better space efficiency on desktop, up to 4 columns
* Nice responsive design
* Less bland look on desktop. The borders were already here but invisible in Forgejo dark theme

Preview:
* https://codeberg.org/attachments/3c9e10ae-3315-46e5-b8bb-8021f6fd8936
* https://codeberg.org/attachments/8196ad89-5ab6-443a-98ce-a70dcc75bca9
* https://codeberg.org/attachments/24f52a14-2ac4-4949-8108-55c34bd3c650

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6795
Reviewed-by: Beowulf <beowulf@beocode.eu>
Reviewed-by: Otto <otto@codeberg.org>

web_src/css/base.css

@@ -18,6 +18,7 @@
   /* other variables */
   --border-radius: 4px;
   --border-radius-medium: 6px;
+  --border-radius-large: 25px;
   --border-radius-full: 99999px; /* TODO: use calc(infinity * 1px) */
   --opacity-disabled: 0.55;
   --height-loading: 16rem;
@@ -1282,10 +1283,6 @@ svg.text.purple,
   width: 100%;
 }
 
-.migrate .svg.gitea-git {
-  color: var(--color-git);
-}
-
 .color-icon {
   display: inline-block;
   border-radius: var(--border-radius-full);