help.markup: added $subsections markup and consolidated $link implementations

help.markup: added $subsections markup and consolidated $link implementations

- Created a new markup element, $subsections, which is a plural variant
     of $subsection. The advantage is that it automatically inserts a
     blank line after the final subsection which makes help articles
     considerably easier to read.
- Consolidated the implementation of $link, $long-link and $pretty-link
- Moved $definition-icons from definition.icons to help.markup
- Moved $pretty-link from help.vocabs to help.markup