feat: add "email settings" feature for admin, previews

docs/glossary.rst

@@ -196,6 +196,20 @@ Glossary
      must be unique across the app, so the correct template files and
      other settings are used when sending etc.
 
+   email module
+     This refers to a Python module which contains :term:`email
+     setting` definitions.
+
+   email setting
+     This refers to the settings for a particular :term:`email type`,
+     i.e. its sender and recipients, subject etc.  So each email type
+     has a "collection" of settings, and that collection is referred
+     to simply as an "email setting" in the singular.
+
+   email template
+      Usually this refers to the HTML or TXT template file, used to
+      render the message body when sending an email.
+
    email type
      The :term:`app` is capable of sending many types of emails,
      e.g. daily reports, alerts of various kinds etc.  Each "type" of