Support webauthn (#17957)

Migrate from U2F to Webauthn

Co-authored-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/less/_form.less

@@ -276,6 +276,10 @@ textarea:focus,
   }
 }
 
+.user.signin.webauthn-prompt {
+  margin-top: 15px;
+}
+
 .repository {
   &.new.repo,
   &.new.migrate,