Removed override templates by moving the content partials to a folder

Updated the main template files to reference the partials/content template parts
Added the content-none partial
Removed the content-gallery, content-image, content-video partials as they are
no longer required. If these need to be displayed differently in future, new
partials can be created as necessary
Added new template for the static front-page
Separated template tags out into a separate file & changed the custom headers

Moved the post thumbnail, paging navigation & hosting functions into
includes/template-tags.php
Created a new includes/custom-header.php file and updated the custom header
support hook to include a load of FreeSpirit header images
Deleted the unnecessary images in the assets/images folder
Added shortcode for displaying a random image