Homepage tweaks for v21.06 release

• We've announced to drop "beta" in the release after v21.03, so let's do that in the header. Let's keep "For Linux enthusiasts.", maybe it looks good if we make Linux green?

While at it, maybe make some design tweaks. I showed the homepage to a designer at some point and got the following feedback, which makes a lot of sense to me:

• header buttons on the top right (blog, download, chats & social media, ...)
  • use the same font size for all buttons
  • don't underline them it's clear that they are buttons
  • make "download" green instead of donate
• header other:
  • add "latest news" to the latest news thing to explain what this is
  • make "latest news" in the style of the buttons above
• footer buttons:
  • don't underline (all of them! right now only "Contribute" is not underlined)
• start page:
  • images of phones: add the names of the phones
  • quotes: don't make them bigger than the headline "What others say"

Implementing these shouldn't be much work.