Refactor fonts dir organization.

fonts/(social_media_icons.* > social_media/social_media.*)
	-Move the different font formats for the `social_media_icons`
	set into their own directory, `social_media/` (also, rename each
	file root to `social_media`).
	-This structure will be adopted for fonts added in the future,
	for a much cleaner `fonts/` directory.

_sass/_mixins.sass
	-Modify the `font()` mixin accordingly.