Additional HTML content drafts

Here comes my first pull request. Hopefully I've gotten it the right way around ;)

Added content drafts for: More Activities, Collaboration, and
Assessment.

Worth noting that in More Activities section, the HTML has been split into moreactivities/turtle/physics to indicate that they will function best as different sections.

Moreactivities 1 and 2 should wrap turtle and physics.

So in paging:
moreactivities1.html
_then_
turtle1
turtle2
etc
_then_
physics1
physics2
etc
_then_
moreactivities2.html

Also, I'd greatly appreciate it if you took a look at the Turtle Blocks content - have I overcomplicated it? Could it be more straightforward or efficient?

And finally, it might be worth breaking out the "assessment-yes" html (where the teacher assessment instructions sit.) Not sure if it's too cramped doing it all at once.