Change how SVG icons are included in pages

Normally you include a .html file containing SVG data to show social
media icons on the page, but this means adding up to 27KB of bloat to
every single page.

With this commit I have moved the SVG data into a .scss file and
converted them into Data URIs so they work on multiple browsers. This
means the CSS file will contain the SVG data rather than each and every
page on the site.