Demo page type and templates

[mgax]

Fix https://github.com/wagtail/wagtail-newsletter/issues/5.

• ~To spin up the site locally, simply run tox -e interactive, it's the same site that is part of the testsuite.~ The demo project is now in the /demo subdirectory.
• The code includes our StreamField migration workaround, to keep the block schema from generating unnecessary migrations.
• I've opted for a simple content schema: one page type with the fields author, date, and a StreamField body.
• The body currently defines block types for rich text, image, and raw HTML, because they seem particularly relevant to newsletter content rendering. I wonder if it makes sense to add anything else at this point. We can easily add more later without generating migration noise.

[mgax]

@thibaudcolas thanks! I've copied the demo site to /demo and restored the previous test site. I've also simplified the configuration and added a note in the README.