Footer updates

[robyngit]

Summary of changes:

• Remove empty divs
• Remove redundant container divs
• Remove unused CSS classes
• Make space for site map
• Add background colour to footer
• Add padding to bottom of cards so that there's space above footer
• Define a maximum content width variable, use for footer content
• Use variables for colours in footer & socials SCSS
• Use BEM convention for naming classes in the footer
• Make the social media links a separate partial, as we may want to re-use this later (e.g. in a contact page)
• Make the social media links configurable in the config Params.yaml file (and document in the README)
• Minor stylistic tweaks