Adding admin file and email template meta box

Set up includes folder with first file: the admin file. Added code to
admin file to add meta boxes, including the email template meta box.
Setup meta box/data to select email template

Setup the “Email Template” meta box and the script to save the meta box
data
Changing slug from 'email' to 'emails'
Added filters file. Filtering 'template_include'.

Added filters file to includes folder. Setup ‘template_include’ filter
to filter the template used for the ‘emails’ pages. If no template is
selected, uses default.php template.