Improve usability and UX

This PR:

• add alt texts to images
• center all logos and make cards have the same height (at home, projects, sponsors, citests)
• change opentechfund logo to svg (it was to pixely)
• remove ghost buttons at /talks
• remove the contribute section at /talks (since it has its page)
• at /talks: move slides to the top (it was lost)

🚨 one link at talks is a 404 We store all of our presentations in PDF or HTML form Is there another one I can add?

Screenshots

talks

citests

home