Move social.toml data file to theme folder so an end user doesn't
accidentally override the theme's social icons with their own social
data. Effectively puts the social.toml data in the theme's namespace.
Add a "social.toml" data file to hold the data for all the social
icons in the footer template. This reduces repetition in the
template code and makes it easier to reuse the data in other
parts of the theme, should that be desired in the future.
It also makes it easier for end users to add new icons by just
overriding the data file, instead of having to override the whole
footer template.
