Add 'contenttype' twig var to listing templates

As suggested from #837

Adds the ability to call {{ contenttype }} to show the name of the contenttype being requested. Useful when the designer would like to show the end-user that no records exist, without hard-coding the name. As such, specifically good for those who do not use content-type-specific listing templates.
Merge pull request #838 from mike-anthony/patch-1

Add 'contenttype' twig var to listing templates