update homepage for v21.03 release

TODO:

• new /download page
• new /chats-and-social-media page (close #138 (closed))
• new /source-code page
  • we have so many git repos now, this page gives an introduction
• tweak navigation (see commit msg / screenshot)
• update text in header from "Calls don't work on most phones yet." to "For Linux enthusiasts."
  • Calls do work on most phones in community and main now.
• new header image (close #136 (closed), calling artist @MartijnBraam... or anybody else got something?)
• css tweaks for new header and responsive view
• update start page
  • add pictures of phones running postmarketOS
  • update text
  • (update quotes?)
• v21.03 blog post (close #135 (closed))
• adjust navigation in cast.postmarketos.org too

Find details in the commit messages. Feedback and reviews welcome!

Screenshots

Pay attention to the new navigation too (top right), and Martijn's new header image in case you did not notice

blog post *** new 2021-03-31 ***

home page *** new 2021-03-31 ***

/download *** updated 2021-03-31 ***

/source-code

/chats-and-social-media

Responsive view before and after

Start page:

Blog post (the slogan used to be only on the start page):