Posts in sample data should belong to category.
Add sidebar_module partial and SidebarPostDecorator class.

Also did some code cleanup in
PagesControllerDecorator::populate_sidebars.

The controller will now populate left and right sidebars (only
left for now,) while the SidebarPostDecorator will provide the
markup for each post that is to be shown in a sidebar.

The plan is to further expand the system with other Sidebar-
Decorators, so each item can be rendered using the same in the
partial.
Title of sidebar entry links to full blog post.
Move markup from decorator to template.

Note to self, use model decorators to massage model data, but leave
the markup to the views where it belong.
Add read more link to sidebar items.
Add application specific locale files for en and nb locales.