navigation: update links
cherry-picked and adjusted from postmarketos.org@499de168
- "src": link to postmarketos.org/source-code/ instead of directly to gitlab
- replace mastodon/twitter/reddit with new "social media" button
- add "download" button
- move buttons around and increase font size in second row, so that now each row is smaller than the row before and so we have a rather big download button (without changing the first row, tried that and it did not look nice)