Provide page turning base template

Provide a base template for views that show a specifc item or set of items and allows navigating to the previous and next items or sets.

• Create new template, derived from base.html
• Expect the following template variables: current_head containing the heading for the current page, url_prev containing the URL for the previous page, url_next containing the URL for the next page
• Render the current_head as a <h2> at the start of the content block
• Render buttons for the previous and next pages
• Provide an empty, inner block that the view itself will then fill its content into (e.g. current_content)
• Use this base template in all currently existing turnable views in all apps