An even better content template to present different content for different users

[CrispinF]

We previously added #578 and it's good, but quite restricted in what you can achieve.
For example there no way of adding some content that will be visible to ALL users.
I think we could do a really powerful template that permits adding any number of blocks of content, each of which can be set to be viewed by either:
All users
Authenticated users
Unauthenticated users
Users in selected roles

And, as for other multi-part templates, it would be nice if the blocks could be re-ordered during editing.