Make email template (short) names more prominent on all email template related views

Show templates' file names in selection list where admins can pick individual templates for customization.

While working on customizing several of the template files to make them GDPR compliant on my local mailman site, I often needed time to identify what template is for what use case. The template file names (TEMPLATES_LIST.keys()) explains the purpose of a template much better and faster than the description (TEMPLATES_LIST.values()) currently does. Thus, I feel it might be helpful to many admins to actually additionally provide the TEMPLATES_LIST.keys() in the templates selection menu.

UPDATE 2019-08-08: I now added another commit that moves the email template's name into the headline (and the template's description into a small-html-tag below the headline). This makes the template views in Postorius much more usable.