Add template snippets useful with panels

* _contact_details.html - use an article to display contact details
* _map_section.html - display a map section
* _navigation.html - display a menu defined in Menuitem model
* _panel.html - display a panel calling snippets defined in Section model
* _panel_contact.html - display a panel which also contains a contact form
* _slideshow.html - display a bxslider slideshow
* base_company.html - base template that can be extended for a company site
                      (extends base_generic.html)
* base_generic.html - define a basic site