Default social link names don't match CSS classes

[tomcornall]

Since the default social link names have Capital Letters: Facebook, Twitter, etc., the class names are not matching as they use lowercase: facebook

This means the social icons don't show.

[tomcornall]

Also, the default links include LinkedIn so this issue will include a replacement icon to cover linkedIn (versus the existing instagram icon)

[tomcornall]

Can be fixed here, by capitilizing these three image classes:

https://github.com/lemonstand/ls2-theme-zest/blob/master/resources/css/style.css#L717