Allow arbitrary nesting in `sidebar.yml`

[felixmulder]

Currently the sidebar allows only one additional level of nesting. This should be changed to allow arbitrary nesting in sidebar.yml for more freedom in page layout.

Currently valid:

sidebar:
    - title: Blog
      url: blog/index.html
    - title: Usage
      subsection:
        - title: Dottydoc
          url: usage/dottydoc.html

Currently invalid, but should be allowed:

sidebar:
    - title: Blog
      url: blog/index.html
    - title: Usage
      subsection:
        - title: Dottydoc
          subsection:
            - title: Dottydoc page 1
              url: usage/dottydoc1.html
            - title: Dottydoc page 2
              url: usage/dottydoc2.html

To achieve the current functionality, we have a split between using toc.html which is included from sidebar.html for rendering, and a custom tag named renderTitle which is implemented in tags.scala.

The current implementation looks at the current path of the page to decide which element to give a CSS show class. When given a show class, the subsection for that element will be visible.