feat(ui): welcome screen for user dashboard (#7030)

It is shown when there's no activity in the feed.

This is a partial implementation of https://github.com/go-gitea/gitea/pull/32990.

Differences:
* drawer icon instead of package icon
* h2 instead of h3
* explore links include a link to organizations list
* explore links are hidden for hidden explore sections
* locales are in JSON, I think it's the time to start using it, the hint is simpler and doesn't lie about following users to get their updates in the feed, which isn't a feature yet
* hint uses general hint color instead of input placeholder color
    * the large icon still uses placeholder color, but I think it's ok

Things to improve later:
* use 24px variant of icon. This will require reworking `tools/generate-svg.js`
* the vue part wasn't ported, but it'd be also nice to have

Inspired-by: Kerwin Bryant <kerwin612@qq.com>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7030
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>

web_src/css/dashboard.css

@@ -79,3 +79,7 @@
 .dashboard .secondary-nav .ui.dropdown {
   max-width: 100%;
 }
+
+.dashboard .help {
+  color: var(--color-secondary-dark-8);
+}