Support simplytest.me

Problem/Motivation

Similar to other projects, we want to use simplytest.me for demo by example.

Proposed resolution

Add demo module with simplytest dependency.

What we should do:
- Define personalisation with subscriber fields like first name, last name
- Add subscribers
-- Some connected with users
--- Some users not yet confirmed (inactive)
-- Some subscribers only
--- Some not yet confirmed (inactive)
-- With personalisation fields filled, some empty
- Add two newsletters
- Configure signup blocks
-- One single signup block
-- One for all newsletters
- Add newsletter issues
-- Sent ones with different subscriber count
-- Unsent ones, at least one for each category
- Add an intro text to front page
- Add maillog to catch mails, immediately display and log them

Remaining tasks

User interface changes

API changes