Remove optional for attributes on input labels

<input> elements that are wrapped in a <label> element have all of the
same functionality and semantics when they don't have a for attribute.
Add a :focus state for menu links
Revert "Remove optional for attributes on input labels"

This reverts commit e5f456fcc7ea6257c602d58485672a056393f0ba.
Merge branch 'menu-focus-state' of git://github.com/Daniel-Hug/pure-site into menu-focus-state
Add hover/focus state to Pure menu heading item
Clean up main.css' formatting and errors