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/svg/gitea-git.svg

@@ -1 +1,3 @@
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