update the styling of checkout and thank you pages

[alicetragedy]

closes https://github.com/rails-girls-summer-of-code/rgsoc-campaign/issues/32 with the following:

• update color of donate button on confirmation page to be consistent with our branding
• remove any trace of old RG-specific fonts and uses our current branding font instead
• improve the copy on both the confirmation and the thank you page
• update all of our URLs to https
• make the footer styling consistent with our current RGSoC footer
• update favicon

see screenshots below.

better fonts on the form page

new color on donate button

new thanks page (with improved footer)

[alicetragedy]

thank you for the review @ramonh <3