i18n: Replace png containing text by svg

To help with the internationalization of main pages, images such as https://github.com/letsencrypt/website/blob/master/static/images/howitworks_authorization.png should be svg, so they can easily be translated

(Additionally, to follow the open-source pledge, ideally all pictures should be accompanied by their sources - a problem partially solved by svg)

Images containing texts:

• Explaining something:

  • https://github.com/letsencrypt/website/blob/master/static/images/howitworks_authorization.png
  • https://github.com/letsencrypt/website/blob/master/static/images/howitworks_certificate.png
  • https://github.com/letsencrypt/website/blob/master/static/images/howitworks_challenge.png
  • https://github.com/letsencrypt/website/blob/master/static/images/howitworks_revocation.png

• Describing:

  • https://github.com/letsencrypt/website/blob/master/static/images/isrg-keys.png
  • https://github.com/letsencrypt/website/blob/master/static/images/le-firefox-chain-of-trust.png

• Graphs:

  • https://github.com/letsencrypt/website/blob/master/static/images/2017.06.28-https-percentage.png
  • https://github.com/letsencrypt/website/blob/master/static/images/HTTPS-Growth-Rate-April-2016.png
  • https://github.com/letsencrypt/website/blob/master/static/images/HTTPS-Growth-Rate-March-2016.png
  • https://github.com/letsencrypt/website/blob/master/static/images/Issuance-April-10-2016.png
  • https://github.com/letsencrypt/website/blob/master/static/images/Jan-6-2017-Cert-Stats.png
  • https://github.com/letsencrypt/website/blob/master/static/images/le-certs-issued-june-22-2016.png

• Paypal: https://github.com/letsencrypt/website/blob/master/static/images/paypal_donate.gif
  That one is should probably stays a gif, and PayPal probably have it in any languages possible. On https://www.paypal.com/us/cgi-bin/webscr?cmd=_button-designer&factory_type=donate it's possible to see all versions with the "customize" link.