Homepage Easter update

Add Easter artwork to the homepage, accompanying the "next week"
Outlasters artwork.

Minor copy update to Giving, as per Copywriting team request.

Changes to be committed:
new file:   images/home/easter.jpg
new file:   images/home/easter@2x.jpg
modified:   index.html
modified:   pages/giving.html
modified:   scss/pages/_home.scss
modified:   stylesheets/app.css