More sidebar, navbar customization. Images.

'hideSidebar' setting renamed 'hideSidebarLinks'. New page
settings: 'sidebarNote', 'hideSidebarNote' and 'navbarItems'.
Template procs for inserting responsive images into pages
(because you can't specify CSS classes in plain
Markdown and may not want to repeat the same HTML markup many
times over). Added 'title' as a shorter synonym for page setting
'pageTitle'.

Improved performance through caching of pageLinks.